So a puzzle with a title of Difficulty – but no indication of its level of difficulty. Nine letters are missing from the subsidiary indications and this then needs to be used to modify the final grid and produce all but one member of a set. Sounds quite hard to me.
Following reading the preamble, the next thing I noticed was the number of clues 72 I make it filling a 14×14 grid. So lots of short answers, which can be some of the trickier clues to solve. But in hindsight, most clues were relatively straight forward.
The additional letters – taken in clue number order, not clue order give MOHS SCALE, which for those with a mis-spent youth like myself, is immediately recognisable as a scale of mineral hardness. A quick refresher, seeing as it is nigh on 20 years since I left the world of Physics behind, yielded the 10 levels of the scale, in ascending order of hardness:
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase Feldspar
7 Quartz
8 Topaz
9 Corundum
10 Diamond

Modified grid fill

Missing item
Orthoclase Feldspar, which at 6 is probably a fair representation of the “Difficulty” / Hardness of this puzzle. Many thanks MynoT for a pleasing puzzle and a chance to reminisce of the pleasurable time I spent studying and researching in physics.
Key:
* Anagram, DD Double definition, Rev. reverse, Underline – Definition in clue
ACROSS
| 1 Control of pipes is holy man’s work (4) | |
| St (holy man as in saint + op (work) = STOP | |
| 4 When 15th of March produces indirect efforts(6) | |
| As (when) + ides (15th March) = ASIDES | |
| M | 8 Symbol of stove (4) |
| DD Aga and Agma = AGMA | |
| 11 Source of watertrembling but not very loud (7) | |
| Aquiver (trembling) – very (v) + f (loud) = AQUIFER | |
| 13 African teachershops in the morning (6) | |
| Mall (shops) + am (in the morning) = MALLAM | |
| 14 Out-of-date financial statementsprepared from deeds held by typesetter? On the contrary (8) | |
| Comp (typesetter) in acts (deeds) = ACCOMPTS | |
| 17 Head accountant left scoundrel(6) | |
| Ras (head) + ca (accountant) + l (left) = RASCAL | |
| 18 Child’s tick’s bigger than this (4) | |
| DD MITE (A mite is defined in Chambers as smaller than a tick) | |
| H | 19 Journalist once recognised not new (4) |
| Ackn. (recognized as in acknowledged? ) – n (new) = HACK | |
| 20 Do not start to pose as a connoisseur (4) | |
| Tarty (to pose?) – t (do not start) = ARTY | |
| S | 22 Girl’s boxer returns (4) |
| Rev Ali (boxer) = ISLA | |
| 24 French aristocrat has money from Laos (gold coin) (5) | |
| Duc (French aristocrat – duke) + at (money from Laos) = DUCAT | |
| 27 Knight almost out of ravine(4) | |
| K (knight) + hors (out of) – s (almost) = KHOR | |
| 29 Confucius keeps algae(4) | |
| Hidden conFUCIus = FUCI | |
| 31 Spanish lady heartlessly produces watery liquids(4) | |
| Senora (Spanish lady) – middle = SERA | |
| 32 Ruler of Czech Republic and Russia primarily (4) | |
| Cz (Czech) + a + r (start of and Russia) = CZAR | |
| 33 False alarm concerning cheek(5) | |
| (alarm)* = MALAR | |
| 35 Mountainsof country without river? (4) | |
| Rural (of ciountry) – r (river) = URAL | |
| 37 Correct gear for areas of shifting sands(4) | |
| (gear)* = AREG | |
| 40 Source of flourin barrel:Uruguay (5) | |
| Drum (barrel) around u (Uruguay) = DURUM | |
| 41 Train of combustible material as safety device (4) | |
| DD FUSE | |
| 42 Turacoof lecturer on railway (4) | |
| L (lecturer) + o (on) + ry (railway) = LORY | |
| 44 Scots gave leave to plunder (4) | |
| DD LOOT | |
| 45 Cast into oil by Dryden(4) | |
| (into)* = OINT | |
| 47 Shakespeare’s pygmy skeleton (5) | |
| DD = ATOMY | |
| 49Learnedseven not five (4) | |
| Seven – v (five) = SEEN | |
| A | 50 Mother’s boy (4) |
| Dam (mother) = ADAM | |
| L | 51 Sesames in tins without name (4) |
| Tins – n (name) = TILS | |
| 53 Suffering in country without sun (4) | |
| Spain (country) – s (sun) = PAIN | |
| 54 Quotefrom a dead dictator (6) | |
| A + d (dead) + duce (dictator as in Mussolini) = ADDUCE | |
| 57 Ten fully composed in easy manner(8) | |
| (ten fully)* = FLUENTLY | |
| 58 Calledthree times when in front of sea (6) | |
| Ter (three times) + med (sea) = TERMED | |
| 59 “Terete” could beincorrect or left out (7) | |
| (incorrect – or)* = CENTRIC | |
| 60 So depart after expression of hesitation (4) | |
| Er (hesitation) + go (depart) = ERGO | |
| 61 Young woman gets hold of uranium base for application to bad purpose(6) | |
| Miss (young woman) around u (uranium) + e (base) = MISUSE | |
| 62 Frame band (4) | |
| DD SASH |
DOWN
| 1 Fish eats one in river(4) | |
| Sar (Fish) around a (one) = SAAR | |
| 2 French bear belonging to us (4) | |
| DD (French word for bear) = OURS | |
| 3 Familiar photoof religious Catholic on Cyprus (5) | |
| Pi (religious) + c (catholic) + cy (cyprus) = PICCY | |
| 4 A very loud song for romantic intrigue(6) | |
| A + FF (very loud) + air (song) = AFFAIR | |
| 5 Enclose marine mammal (4) | |
| DD SEAL | |
| 6 Lawyer and spies in ancient country annexed by Rome(5) | |
| DA (lawyer) + Cia (spies) = DACIA | |
| 7 Show joy, perhaps, on return of English book (5) | |
| Rev (E (English) + Tome (book)) = EMOTE | |
| 8 Character not active in sacred river(4) | |
| Alpha (Character) – a (active) = ALPH | |
| 9 Cosmetic bleat about blemish (7) | |
| Maa (Bleat) around scar (blemish) = MASCARA | |
| O | 10 Violently angryFrenchman in Alaska (4) |
| Ak (alska) around M (Frenchman) = AMOK | |
| 12 Cheese particle (5) | |
| DD QUARK | |
| 15 Lucerne for example physician rarely used(5) | |
| DD MEDIC | |
| 16 Bit of skirt, be silent (4) | |
| DD TACE | |
| 21 Nameless chief for those in Perth(4) | |
| Thane (chief) – n (nam-less) = THAE | |
| 23 Son and father mimic Farinelli, for instance(5) | |
| S (son) + pa (father) + do (mimic) = SPADO | |
| 25 Making up panel for more than one story(5) | |
| Rev. Slat = TALES (There would appear to be a missing E in the subsidiary indication) | |
| S | 26 Contralto having fatty acid in aural passage (5) |
| C (contralto) + ala (fatty acid – alphalinolenic acid) = SCALA | |
| 28 Drunken revelor heartless gaiety (4) | |
| Or + gy (heartless gaiety) = ORGY | |
| 29 Infectionfrom down mostly (3) | |
| Flue (down) – e (mostly) = FLU | |
| C | 30 Raw and impolite (5) |
| DD Crude and Rude = CRUDE | |
| 31 Mystic’s rising whenever visiting America (4) | |
| Rev (if (whenever) + us (america)) = SUFI | |
| 34 In backward Argentina tango is magic(3) | |
| Rev. RA (Argentina)) + t (tango) = ART | |
| 36 Listleaving nothing out of series of calls (4) | |
| Round (series of calls) – o (nothing) = RUND | |
| 38 Rower’s wife becomes fit to bring back bits of plankton(7) | |
| Rower – w around Rev,. (fit) = ROTIFER | |
| 39 Strip backof insect taking exercise (5) | |
| Fly (insect) + pe (exercise) = FLYPE | |
| 43 Reported part of aeronautical manoeuvre(4) | |
| Homoym of Role (part) = ROLL | |
| 45 Spirit in watermakes a noise in drink (6) | |
| One (Drink) around din (noise) = ONDINE | |
| 46 Swordof soldiers with lifeforce (5) | |
| Ta (soldiers) + Chi (Life force) = TACHI | |
| 48 International with independence that’s us in WI(5, hyphenated) | |
| I (international) + and (with) + I (independence) = I-AND-I | |
| 49 Before group’s return note venues (5) | |
| Si (note – variant of te) + Rev. (Set) = SITES | |
| 50 Passagesof a poem once special (5) | |
| A + Dit (poem once) + s (special) = ADITS | |
| 51 Lecturer to abandon right of ownership at once(4) | |
| Tuter (Lecturer) – r (right) = TUTE | |
| 52 Wrestlingwith problem oddly at first (4) | |
| Sum (problem) + o (oddly at first) = SUMO | |
| E | 54 Agreements, say, must be modified (4) |
| (say)* = AYES | |
| 55 Man in ancient city on a lesser pilgrimage(4) | |
| Ur (ancient city) around m (man) + a = UMRA | |
| 56 Eatsomething extra hot (4) | |
| Etc (something extra) + h (hot) = ETCH |
twencelas
there was one extra difficulty that eluded me and also perhaps eluded you – the preamble says that 14 answers must be entered with a single letter misprint. You explain here 9 of them. which are the other 5 and what words do they make?
Maestro
Don’t forget the crossing entries.
Fulll list below:
Asides = AZIDES, Seal = ZEAL, Quart = QUARK, Khor = THOR, Scapa = SCALA, pory = LORY, atomy = ATOPY, tute = TITE, sites = RITES, seen = REEN, medic = MEDIA, tace = TACO, sera = SORA tales = TALCS making 14 and the usual telegraph gremlin mite = CITE giving 15.
I don’t think 20A works, unless the word play is something else. And it’s always the Oxford English Dictionary, not ODE, unless that stands for something else or it’s yet another EV error.
Tony – I’m not happy with 20ac certainly
I read ODE as OED – suspect someone from the grauniad reviewed it, but it does nt refer to the 20ac entry
for 20, do = PARTY, not start to Pose leaves ARTY.
ODE is fine – Oxford’s equivalent to the single-volume Chambers is called the Oxford Dictionary of English (hence ODE) while the big mega-volume set (not I think to be
published in hard copy again) is the OED.
Thanks Phi for the clarification – after my comment I noticed that the ODE was also referred to in this week’s Inquisitor so suspected as much.
bb – Thanks for putting me out of my misery with 20ac
I think 25a is a double definition. Per Chambers ‘tales’ = the filling up, from those who are present, of a deficiency in the number of jurors.
Sorry, that should be 25d.
Thanks Deep Throat I agree.