14 entries without definition but some form of connection leading in one direction to an unclued entry. This unclued entry is repeated elsewhere in the grid. 20 cells to highlight therefore either a 10, 5 or 4 letter word I would think.
The unclued entry at 4ac confirmed the unclued entry to be 4 letters. So five occurrences.
The clues were very straight forward and the undefined entries started to show a pattern of ways to get somewhere (highlighted in red below).
41 ac being ROME as in the phrase “All roads lead to Rome”
And that was that.
Probably my quickest solve for a while but fun along the way.
Thanks Gnomie
Key: Rev. reverse; * anagram; DD double definition; underline definition
Across
1 Pale when hot and, in the end, dry (4)
As (when) + h (hot) + y (in the end dry) = ASHY
4 Mariners’ skills to observe, we’re told, in essentially low?cost, flat boats (9, two words)
Homonym of see (to observe) + c (middle of low-cost) + rafts (flat boat) = SEA CRAFTS
12 Capital old-style lovers (6)
Rome (capital) + os (old style) = ROMEOS
13 Expression of disapproval used primarily for dancer’s skirt (4)
Tut (expression of disapproval) + u (used primarily) = TUTU
14 Acting in front of stadium (6)
A (acting) + venue(stadium) = AVENUE
16 Uttered statement of intention to dwell in isolated place (4)
Homonym of I’ll (statement of intention) = ISLE
18 Company boy returning after recurrent sex with simultaneous highs (7)
Co (Company) + Rev.( lad (boy) + It (sex) ) = COTIDAL
19 African-American woman not accepted to stay in Scotland (4)
Sista (African-American woman) – a (accepted) = SIST
20 In favour of male type of performance (4)
Pro (in favour of) + m (male) = PROM
24 Shred leaves half of Indian coinage? (6, two words)
Tea (leaves) + rupees (-ees – half of Indian coinage) = TEAR UP
25 Casual provisions US agents briefly rejected (5)
Rev. Narcs (US agents) = SCRAN
26 Resilient castle coming to grief while staying independent (7)
(castle)* around I (independent) = ELASTIC
28 Everybody look on – no escape ultimately (5)
All (everybody) + eye (look on) – e (no escape ultimately) = ALLEY
31 A feeling of fear without end about criminal conspiracy of silence (6)
Rev. A + tremor(feeling of fear) – r (without end) = OMERTA
33 Swears regularly with bit of bloodthirsty language (4)
SwEaRs + b(bit of bloodthirsty) = SERB
35 Make acceptable south-eastern lines (4)
SE (south-eastern) + ll (lines) = SELL
36 Anglo-Irish darling on the land around Troy (7)
Ashore(on the land) around t(troy) = ASTHORE
38 Desire what idols sumptuously have for starters (4)
Wash Idols Sumptuously Have (for starters) = WISH
39 Once besmirched, for instance, money with old till (6)
As (for instance) + m (money) + ear (old till as in plough) = ASMEAR
42 Play on words is principally held to inflict suffering (6)
Pun (play on words) + is + h (primarily held) = PUNISH
43 Surprisingly modern ape (9)
(modernape) = PROMENADE
44 One’s different ages (4)
(ones)* = EONS
Down
1 Academic note over deplorable skills on the rise (11)
A (academic) + ut (note) + o (over) + Rev. (sad (deplorable) + arts (skills)) = AUTOSTRADAS
2 Diets duly organised in well-considered manner (9)
(diets duly)* = STUDIEDLY
3 Anyone involved in pay-out (3)
Hidden paY-OUt = YOU
4 Unusually to scrape local oyster bed (5)
DD SCALP
5 Not many in Dundee or in Dunblane (3)
Hidden dunblANE = ANE
6 Utterly defeat English (5)
Rout (utterly defeat) + e (English) = ROUTE
7 Innermost being in Paris of aged grasping male (3)
Ae (aged) around m(male) = AME
8 Foster Queen (6)
Feed (foster) + er (queen) = FEEDER
9 Army acquiring larger than normal fighting dog (4)
TA (army) around Os (larger than normal) = TOSA
10 Physical therapy is a new medicine (6)
Pt (physical therapy) + is + a + n (new) = PTISAN
11 Equal portions swapped in Jewish feast (8)
Passover (Jewish feast) swapping first 4 with last 4 = OVERPASS
15 Disyllable noun rewritten (11, two words)
(disyllable + n)* = BLIND ALLEYS
17 Holy man consuming ash for example (6)
St (holy man) around tree (ash for example) = STREET
18 Subdue spirit of Perth’s rather objectionable person (3)
DD COW
21 Short time on hill with a Yankee (8)
Mo (short time) + tor (hill) + w(with) + a + y (yankee) = MOTORWAY
22 Finished tax after account (6)
Acc(account) + ess (tax) = ACCESS
23 Camel tent rigged for herders (9)
(camel tent)* = CATTLEMEN
27 No medicinal plant raised for old judge in Ireland (6)
Rev. (No + herb (medicinal plant)) = BREHON
29 Characters brought up among Boston Italians might be designated thus (6)
Rev. hidden (bostON ITALians) = LATINO
30 Changing direction of sheltered side for sheep enclosure (3)
Lee (sheltered sde) swapping R for L (changing direction) = REE
32 Waterfall over base of corrie (5)
Foss (waterfall) + e (base of corrie) = FOSSE
34 Upset wife the French ply with drink (5)
Rev. Rib (wife) + le (the French) = BIRLE
37 Saint not entirely chaste (4)
S (saint) + pure (chaste) – e = SPUR
39 Try Maoism from time to time (3)
mAoIsM = AIM
40 Important date half rubbed out (3)
Erased (rubbed out) – sed (half) = ERA
41 Currency almost used up (3)
Rev. Euro (currency) – o = RUE