With only two entries during my first (quick) pass through the clues I thought this one was going to be tricky. However, I managed to get a foothold in, and complete, the SE corner and then spread out from there, making steady, if relatively slow, progress until the grid was complete.
As is often the case, I did wonder why I didn’t get some of the clues earlier because, with the benefit of hindsight, they did seem obvious. Perhaps I was just having an off day.
Across
1 Southern wave-top to deliver fish (8)
SCOMBRID – S (southern) COMB (wave-top) RID (deliver)
7 Poet‘s line swallowed by fish (5)
SCALD – L (line) in (swallowed by) SCAD (fish)
12 Old train / bore (4)
TIRE – double def.
13 Source of gas taking a long time to run (7)
AERATOR – A ERA (long time) TO R (run)
14 Source of motivation always keeping singer active (9)
ENERGISER – EER (ever) around (keeping) an anagram (active) of SINGER
15 Long passage linked to large central region (5)
NAVEL – NAVE (long passage) L (large)
16 Put down fibres as a show of respect (5)
KOTOW – KO (put down) TOW (fibres)
17 Moon showing English sailors beside slope behind rice field (6,7)
PADDY’S LANTERN – PADDY (rice field) SLANT (slope) E (English) RN (sailors)
22 Crooked window, unsightly thing around a dwelling, not once overlooked (8,5)
BATEMENT LIGHT – BLIGHT (unsightly thing) around TE[ne]MENT (a dwelling, not once overlooked)
23 Encouraged to restrain King’s desire (5)
GREED – GEED (encouraged) around (to restrain) R (king)
25 Spirit shown by boisterous gathering with noisy instrument (5)
KAZOO – KA (spirit) ZOO (boisterous gathering)
27 One in no hurry to become National Treasure, possibly (9)
SAUNTERER – an anagram (to become … possibly) of N[ational] TREASURE
28 Standard presentation of university on radio? (7)
UNIFORM – U (university) is UNIFORM in the phonetic alphabet (presentation of … on radio)
29 Old relative taken aback about one of the animals (4)
ZOIC – COZ (old relative) reversed (taken aback) around (about) I (one)
30 Hefty competition? Pah! (5)
BEEFY – BEE (competition) FY (pah)
31 Christian group retaining power, not at the highest level (3-5)
TOP-NOTCH – TOC H (Christian group) around (retaining) P (power) NOT
Down
1 Stop the race (4)
STEM – double def.
2 Bright red tavern, a drinking place seen in description of film? (11)
CINNABARINE – INN (tavern) A BAR (drinking place) in (seen in) CINE (description of film)
3 Study probing nucleus of most mythological creatures (6)
OREADS – READ (study) in (probing) [m]OS[t] (nucleus of most)
4 Say one gets upset, tucking into extra fish (6)
BIGEYE – EG (say) I (one) reversed (gets upset) in (tucking into) BYE (extra)
5 Bar bouncer knocked over (4)
RAIL – LIAR (bouncer) reversed (knocked over)
6 River bird seen in host (6)
DEEJAY – DEE (river) JAY (bird)
8 Yam? Caught with a small piece (6)
CAMOTE – C (caught) A MOTE (small piece)
9 A hack in Congress (2,2)
AT IT – A TIT (hack)
10 Design re: getting involved with choir is using too many words (11)
LOGORRHOEIC – LOGO (design) plus an anagram (getting involved) of RE CHOIR
11 Keep turning up clutching crude fishing accessory (4-3)
DRAW-NET – TEND (keep) reversed (turning up) around (clutching) RAW (crude)
13 Fern spaniel trashed with little hesitation (9)
ASPLENIUM – an anagram (trashed) of SPANIEL plus UM (little hesitation)
17 US town bowled over after turning up tavern food (3,4)
PUB GRUB – UP reversed (turning) plus BURG (US town) B (bowled) reversed (over)
18 Rising charges incurring old fellow’s reprimand (3,3)
SEE OFF – FEES (charges) reversed (rising) around (incurring) O (old) F (fellow)
19 Walker brought up gallery contents – go (6)
DEPART – PED (walker) reversed (brought up) ART (gallery contents)
20 Clear mountain climbing may reveal glassy surface (6)
PLATEN – NET (clear) ALP (mountain) reversed (climbing)
21 Past corruption leads to change in Jerusalem (6)
AGOROT – AGO (past) ROT (corruption)
24 Lake in sequence, neither first nor last (4)
ERIE – [s]ERIE[s] (sequence, neither first nor last)
25 Local hillock – a good bet to be surmounted by knight (4)
KNAP – K (knight) NAP (a good bet)
26 Tree out of line in gateway (4)
ARCH – [l]ARCH (tree out of line)
Thanks for this. All I can remember of this puzzle is spending ages thinking 22ac must be BASEMENT LIGHT but not seeing why. (I couldn’t find such a phrase in Chambers and googling only got offers for basement light fittings.) Can’t remember now how the penny dropped but I’ve got a lot of doodlings of the letters around the grid. Also can’t remember now if I’d finished this by the time the Super Bowl started.