Fifteensquared

Here’s what I like: a puzzle that is challenging but so good I finished it fairly easily. For all that, I fail to fully understand 7D.

Across
1. SPECIMEN - S (small) + MICE (quiet things) backwards in PEN (fold). Oh what a devious clue!
6. SHANTY - anagram of HASNT + Y (yes)
9. SPRAIN - R[ooney] in SPAIN (country)
10. PALATINE - PA (father) + LATIN (old Roman) + [sit]E
11. HEADMASTER - MAST (post) in HEADER (shot)
12. POOP - palindrome
13. COURSE - homophone
15. ANTELOPE - anagram of NEAT + LOPE (stride)
18. LISTLESS - double/cryptic definition
20. TITBIT - TIT (bird) + BIT (pecked)
21. SHOE - O[xford] in SHE (woman)
23. SMALL TIMER - anagram of MILLSTREAM
25. HEADGEAR - anagram of EG HAD + EAR (lateral feature)
26. SPIDER - double definition. A spider is a type of rest used in snooker and billiards.
27. WARM UP - double definition
28. WAR CHEST - ARCH (chief) in WEST (quarter)

Down
2. PEPPERONI - PEPPER (spicy stuff) + ON (on) + I[talian]
3. CHARD - homophone
4. MANGANESE - MAN (chap) + AG (silver) backwards + anagram of SEEN
5. NAPHTHA - NAP (pile) + H (hydrogen) + THA[t]
6. SOLAR - L (line) in SOAR (fly)
7. AUTOPILOT - AU (gold) + TO (to) + PI (religious?) + LOT (Biblical character). Does “religious” clue PI and, if so, how?
8. TANGO - GNAT (pest) backwards + O (love)
14. ROTTERDAM - OTTER (river animal) + D (died) in RAM (force)
16. TOTALISER - anagram of ARISTOTLE
17. PRICELESS - P (Penny) + RICE-LESS (missing paella ingredient)
19. SPARROW - PARR (fish) in SOW (pig)
22. HYENA - YEN (longing) in HA (laugh). Yes!
23. STEEP - double definition
24. IRISH - IRIS (flag) + H[ibernian]