In the asterisked clues the wordplay for the grid entry gave a Spoonerism of the answer suggested by the definition. The normal clues were for the most part (too) easy, though some (presumably) unintentional added difficulty was provided by incorrect enumeration in three clues.
The ‘definition’ answers are given in blue.
Across
1* BEATEN CUSTER BEATEN (defeated) CUSTER (US battle hero) – Buster Keaton (silent star)
11 SOWAR SO (thus) WAR (battles etc.)
12 BAAL BAA[s] (African overseer without son) L (left)
13 RATA RAT (desert) A (one)
14 ERAS [v]ERA’S (girl’s not very)
15 XIAN X (cross – in short) IAN (chap)
16 INHERE IN (alight) HER (lady) [v]E[stas]
18 DEC D[aylight’s] E[arly] C[hill]
20* SHED RIFT SHED (slough) RIFT (split) – red shift (effect due to receding light)
22 GAJOS JAG (posh car) reversed OS (very big)
25 LAUNCH A (one) in LUNCH (snack)
26* SEA RAT (pirate) – resat (took exam again) – wrong enumeration, Chambers gives this as a single word
27 SEGUE hidden in ‘chineSE GUEsts’
30 DAH HAD reversed
36 GENTOO GEN (information) TOO (also)
37 RAIL dd – wrong enumeration
38 HEIR HE (man) I (one) R (right)
39 PERU hidden in ‘old-tyPE RUins’ – wrong enumeration
40 KNOT TONK (strike) reversed
41 ENROL *(LONER)
42/32* DOGGING A FLED HORSE cd – flogging a dead horse (futilely)
Down
1* BORING SCORED BORING (tedious) SCORE (reckoning) D (depth) – scoring board (where would you see the points?)
2 ELAND E (Eastern) LAND (terrestrial)
3/28* AS THE CRY FLOWS cd – as the crow flies (in a straight line)
4 EWERS SEWER with S moved to end
5* NAREHET NARE (hawk’s nostril) HET (heated in some parts) – hairnet (container of locks)
6 CRAW C (Cuba) RAW (immature)
7 SAXE homophone of ‘sax’ (chopper)
8/31* TAILING A TELL TAILING (following) A TELL (famous archer) – telling a tale (narrating the story)
9 ELAN ELAN[d] (2 after tail’s trimmed)
10* REND THE BOUND REND (break) THE BOUND (limit) – round the bend (bonkers)
17 ODAS hidden in ‘pagODA Say’
19 CORF CORF[u] (island’s bottomless)
21 RUED homophone of ‘rude’ (impolite)
23* JAY BING JAY (bird) BING (heap) – Beijing (famous city)
24 SALE S (special) ALE (beer)
25* LAWGING LAW (statute) GIN (drink) G[et] – galling (really annoying)
29/35* SERRA KEEN SERRA (saw) KEEN (strong) – kerosene (jet fuel)
33 MAKO MA (mother) OK (alright) reversed
34 FLOG GOLF (club game) reversed