A mainly straightforward solve, with just a couple of clues at the end requiring a bit more rumination. I especially liked 13dn for its mental imagery! Most seemed very succinct clues, too, with economical indicators and definitions - a style I like. There’s something about not knowing which word of a three-word clue is the definition, and having to hold both possibilities in mind, that I find enjoyable (for example 12ac). Of course, then 17ac comes along and forces a change of gear!
Definitions underlined.
| Across |
| 1 | Mark the moment? (4) |
| TICK - double definition. |
| 3 | Element in ringtone that's phony? (8) |
| NITROGEN - anagram of (phoney) RINGTONE. |
| 9 | Country song gripping people (7) |
| ARMENIA - ARIA (song) around (gripping) MEN (people). |
| 10 | Sub turned about (1-4) |
| U-BOAT - anagram of (turned) ABOUT. |
| 11 | Sauce sadly backfiring, tasteless ultimately inside (5) |
| SALSA - ALAS (sadly) backwards (backfiring), with last letter of tastelesS inside. |
| 12 | Quietly send favour (6) |
| PREFER - P (quietly) and REFER (send). |
| 14 | Epic thief goes wild for old coins (6,2,5) |
| PIECES OF EIGHT - anagram of (wild) EPIC THIEF GOES. |
| 17 | A meaty firework? (6) |
| BANGER - cryptic definition. |
| 19 | Comfortable thing, right up to this point (2,3) |
| SO FAR - SOFA (comfortable thing) plus R (right). |
| 22 | Greek character has not finished passing mountain (5) |
| ALPHA - HAs (not finished, i.e. without the final letter) after ALP (mountain). |
| 23 | Plant, one no men adored, partly rejected (7) |
| ANEMONE - hidden in (partly) onE NO MEN Adored, reversed (rejected). |
| 24 | Comparatively fair offer in the end — less important to accept that (8) |
| PRETTIER - PETTIER (less important) to accept last letter of offeR (in the end). |
| 25 | Girl— one teacher knocked over (4) |
| IRIS - I (one), with SIR (teacher) backwards (knocked over). |
| Down |
| 1 | Part of a church where patterns formed (8) |
| TRANSEPT - anagram of (formed) PATTERNS. |
| 2 | Beast arrived late at the start (5) |
| CAMEL - CAME (arrived) plus first letter of Late (at the start). |
| 4 | Trendy to steal? That's improper! (13) |
| INAPPROPRIATE - IN (trendy) and APPROPRIATE (steal). |
| 5 | Convincingly defeat on English course (5) |
| ROUTE - ROUT (convincingly defeat) on E (english). |
| 6 | Brick— one won’t float? (4,3) |
| GOOD EGG - double definition. A brick is a reliable person, and only bad eggs float in water. |
| 7 | Fruit— bananas? (4) |
| NUTS - double definition. |
| 8 | Incense in vinegar nearly up-ended (6) |
| ENRAGE - hidden in vinEGAR NEarly, backwards (up-ended). |
| 13 | Naked king pinched by lecherous types? (8) |
| STARKERS - K (king) surrounded (pinched) by STARERS (lecherous types). |
| 15 | Specimen is old enough (7) |
| EXAMPLE - EX (old, former) and AMPLE (enough). |
| 16 | End of June, flower festival (6) |
| EASTER - last letter of junE with ASTER (flower). |
| 18 | Allow elderly relative time (5) |
| GRANT - GRAN (elderly relative) and T (time). |
| 20 | Sounds like a mistake on which to stand? (5) |
| FLOOR - sounds like “flaw” (mistake). |
| 21 | Insect used to be quiet (4) |
| WASP - WAS (used to be) plus P (quiet). |