Financial Times 13,837 by Mudd

Prize puzzle from the Weekend FT of October 29

I think this is an exceptionally fine Mudd puzzle with many strong clues, my favourites being 1A (SAINT-SAENS), 26A (DUNG), 18D (PRESENT) and 21D (CASINO). I also like 15A (ARMY) and 5D (ETERNAL TRIANGLE). And I am baffled by 9A (ATOM?). I trust someone will tell me how it works.

Across
1. SAINT-SAENS – anagram of NASTINESS
7. BUST – double definition
9. ATOM ???. I figure the definition here has to be “particle”, ATOM the answer and “Chambers halved?” the wordplay. But I can make no sense of “Chambers halved”. The capital suggests a reference to Chambers dictionary although that spoils the surface reading. But no matter how I interpret “Chambers”, I get nowhere! (See comments below for the truth!)
10. OPEN LETTER – OPEN (straightforward) + LETTER (character)
11. NEEDLE – double definition
12. NECKLINE – NECK (make out) + LINE (occupation)
13. CONJUGAL – NO (no) backwards + JUG (prison) both in CAL (state)
15. ARMY – double definition
17. SPAT – double definition
19. ROULETTE – LET (obstacle) in ROUTE (the way)
22. DEMERARA – A (a) + RARE (bloody) + MED (large expanse of water) all backwards
23. LESSON – “less on” (more off)
25. LEFT-WINGER – LEFT (dumped) + WINGER (bird)
26. DUNG – hidden word
27. STAR – double definition
28. OVEN GLOVES – V[enison] in anagram of GONE + LOVES (adores)

Down
2. ART DECO – anagram of TRADE + CO (business)
3. NOMAD – NO (no) + MAD (nuts)
4. STONE-AGE – TONE (colour) in SAGE (a shade of green)
5. ETERNAL TRIANGLE – anagram of TALLER ARGENTINE
6. SPLICE – L[ass] in SPICE (excitement)
7. BATTLE-AXE – double definition
8. STERNUM – STERN (back) + [b]UM (bottom, not top)
14. JET-SETTER – JET (black) + SETTER (dog)
16. BULLFROG – LL (two lakes) + anagram of FOR both in BUG (insect)
18. PRESENT – P[ast] + R (right) + anagram of TENSE
20. TROUNCE – [per]T + [senio]R + OUNCE (cat)
21. CASINO – C[antonese] + A (a) + SINO (Chinese)
24. SADDO – ADD (tot) in SO (very)