Welcome to Hawk – new to the Inquisitor but not to the Listener.
Preamble: All clues are normal. Each Across answer must pass a letter to the answer which follows it to form the next entry, i.e. from 1ac to 5ac, from 5ac to 9ac etc, the answer to 38ac passing a letter to the entry for 1ac. All transformations leave real words or phrases. Starting at a place and in a direction to be determined, passed letters spell the theme, to be illustrated by highlighting six words in alternate rows, some of which are examples of their type (22 cells in all). Answers to the across clues at 9, 16 and 37 are in Collins.
Having read the preamble I was somewhat apprehensive, and it seemed natural to start on the down clues. First time through I solved only about a quarter, but including the rather useful 2d EPHAH, 3d LUNGFUL, and 10d GOEL. On to the acrosses: 9a was PLUE to be entered as PLUG having passed the E to 11a and received a G from 5a GOINGS, probably the first; and 15a was MANN entered as MANG having donated an N and inherited the G. Progress was slow – the clues were tough, especially as there was a lot of cold-solving to be done, but they were good & very fair. 
I found myself in a bit of a tangle with 16a: the answer was COURIE (or COORIE) and it had to lose a letter & receive an N from 15a but that didn’t mesh well with the intersecting down entries … until I realised that an incoming letter wasn’t necessarily a direct replacement for the outgoing one: in this case COURIE had to pass on the I, shuffle a couple of letters across, and pop in the N between the O & the U to give CONURE.
I could make OF BEING from the passed letters of answers to 27a back through 14a (obviously not destined for The Unbearable Lightness of Being) and after some more solving I had enough to Google THE GREAT CHAIN OF BEING that “begins with God and descends through angels, humans, animals and plants to minerals” (thank you Wiki).
With the passed letters in hand, I solved the remaining clues bar 38a – the only words I could find to fit the grid & the rubric were COSTER becoming CO-STAR, but that was clearly wrong! Anyway, on to the endgame. It wasn’t too difficult to pick out GOD ANGEL MAN WOLF ASH in the even-numbered rows but where was the mineral in the bottom row? Aha – it had to be SPAR and the resolution to 38a was CASPER becoming CASPAR. Phew! What a workout. But worth it.
Thanks, Hawk. The grid construction is an absolutely marvellous piece of work.
| No. | Clue | Answer | Entry | Wordplay |
| Across | ||||
| 1 | Mottled horse just about under control (6) | MARLED | MEALED | MAR(e) (horse) LED (under control) |
| 5 | Behaviour when day’s good for departures (6) | GOINGS | O-RINGS | DOINGS (behaviour) with D(ay) replacing G(ood) |
| 9 | Prince with uncommon syphilis cut Victoria’s old fur (4) | PLUE | PLUG | P(rince) LUE(s) (syphilis) |
| 11 | Short muscle man in the frame (5) | LATHE | ELATE | LAT (muscle, latissimus dorsi) HE (man) |
| 13 | Applied wrapping without frills and dispatched (4) | SENT | SHEN | SET (applied) around (a)N(d) |
| 14 | Start to broadcast end of lasting peace (6, 2 words) | GO LIVE | OLIVET | (lastin)G OLIVE (peace) |
| 15 | Novelist consumed by human nature (4) | MANN | MANG | (hu)MAN N(ature) |
| 16 | Caught chill in Thurso? Snuggle up there (6) | COURIE | CONURE | C(aught) OURIE (chill, Scot) |
| 18 | Spotted, unruly picketers fled, abandoning working pits (9) | FLECKERED | FLICKERED | [PICKETERS FLED]* ¬ [PITS]* |
| 19 | Dances, expressing advanced curves (6) | RHUMBS | RHEUMS | RHUMBAS (dances) ¬ A(dvanced) |
| 25 | Bars for sailors housing railroad WC, say (6) | FIELDS | BIELDS | FIDS (bars used on ships) around EL (railroad) |
| 27 | Pay court to record damaged sack bales (9) | WOOLPACKS | WOLFPACKS | WOO (pay court to) LP (record) [SACK]* |
| 30 | Call to report syringing bottles (6, 2 words) | RING IN | ORIGIN | (sy)RINGIN(g) |
| 32 | Twice-peeled masala shrimp in which Indian may be dressed (4) | SARI | SNAR | (ma)SA(la) (sh)RI(mp) |
| 33 | Withdrew, Roderigo’s opening parts cut (6) | SHRANK | SHRINK | R(oderigo) in SHANK (cut) |
| 34 | Numbers leaving caff to praise from poet (4) | HERY | AERY | NOSHERY (caff, sl) ¬ NOS (numbers) |
| 35 | Change ensign after docking (5) | PENCE | PHENE | PENCE(l) (ensign) |
| 36 | Regularly turns into mole, for instance (4) | UNIT | UNCI | (t)U(r)N(s) I(n)T(o) |
| 37 | Salvation Army improvised on cello-like instrument (6) | SARODE | STRODE | SA (Salvation Army) RODE (improvised) |
| 38 | Piece of cake, according to friendly spirit (6) | CASPER | CASPAR | C(ake) AS PER (according to) |
| Down | ||||
| 1 | Past master abruptly hired fellow German physician (6) | MESMER | MES (master, obs) MER(c) (hired fellow) | |
| 2 | Handouts initially in heap reordered for good measure (5) | EPHAH | H(andouts) in [HEAP]* | |
| 3 | Lines about topless martial art a great inspiration (7) | LUNGFUL | LL (lines) around (K)UNG FOO (martial art) | |
| 4 | Rather odd when queen goes over-dressed (7) | DOLLISH | DROLLISH (rather odd) ¬ R (queen) | |
| 5 | Lyrical as I, but not I (4) | ODIC | IODIC (as I, iodine) ¬ I | |
| 6 | Recall always turning up to eat well (6) | REVOKE | EVER< (always) around OK (well) | |
| 7 | Former fools: they need no instruction (8) | NATURALS | double definition | |
| 8 | Stuck-up headteachers oddly cancelled break (6) | SECEDE | (h)E(a)D(t)E(a)C(h)E(r)S< | |
| 10 | Truth disheartened Hebrew blood avenger (4) | GOEL | GO(sp)EL (truth) | |
| 12 | No longer tear up or maintain fences (6) | TO-REND | OR in TEND (maintain) | |
| 17 | One with strong feelings about Israel misbehaving (8) | REALISER | RE (about) [ISRAEL]* | |
| 20 | Expression of joy a difficult thing in 622 AD! (6) | HOORAH | HOOR (a difficult thing) AH (anno Hegirae, in the year of Hegira, 622 AD) | |
| 21 | Connecting at summit, alumnus admired leaving institute (7) | OBCONIC | OB (old boy, alumnus) ICONIC (admired) ¬ I(institute) | |
| 22 | Gordon’s dogs eat a non-smoker (7) | MESSANS | MESS (eat) A NS (non-smoker) | |
| 23 | Rushes answer, essentially my mistake (6) | SWOOPS | (an)SW(er) OOPS (my mistake) | |
| 24 | Coiled jumping spider (6) | SPIRED | [SPIDER]* | |
| 26 | Rose: skipping adult with more agility (6) | SPRYER | SPRAYER (rose) ¬ A(dult) | |
| 28 | Monarchy twice elevated NZ resident (4) | KUKU | UK×2 (monarchy) rev. | |
| 29 | Girl in distant flats viewed through telescope (5) | MARIA | MARIA (level areas on the Moon or Mars) | |
| 31 | Cardinal rebuffed evening quarters (4) | NINE | (ev)ENIN(g) rev. | |