Financial Times 12,989 by Cinephile

Prize puzzle from the FT Weekend edition of January 31
I found this puzzle rather easy but fancy it might not be for those less familiar with mathematical terminology. The combination of 13 and 15A is rather good.

Across
1. CACKLE – as in “cut the cackle”
4. SCUPPERS – double definition
9. ABRADE – homophone (“a braid”)
10. ALL RIGHT – double/cryptic definition
12. CHIP – double definition
13. SIXTH SENSE – anagram of HENS SEXIST
15. ANGEL OF DEATH – anagram of HEN FATALE GOD
18. FOURTH ESTATE – O[ne] in FURTHEST (most distant) + [w]ATE[r]
21, 17. COORDINATE GEOMETRY – COORDINATE (get) + GEO (small boy) + ME (setter) + TRY (attempt)
22. SCAM – hidden word
24. ROMANTIC – MAN (male) in [e]ROTIC
25. VARLET – anagram of TRAVEL. But this suggests that “agent” must be the anagram indicator and I do not see how it can serve that role. Am I missing something?
26. ENTIRELY – ENT (hospital department) + I (I) + RELY (depend)
27. TRUSTY – T (time) + RUSTY (out of practice)

Down
1. CHARCOAL – CHAR (cleaning lady) + CO (company) + AL[l]
2. CARRIAGE – double definition
3. LIDO – LID (cover) + O (nothing)
5. CALL THE SHOTS – double definition. I started out thinking this might be “rule the roost” as it fit with the cross letters I had but it sure did not fit with the wordplay.
6. PARASITISM – anagram of PAIRS A MIST
7. ENGINE – double definition. Bit of a liberty here perhaps as one talks of searching on a search engine but not on, simply, an engine. Besides which, “search” plays a double role.
8. SETTER – cryptic definition. I seem to remember seeing this clue before.
11, 16. DIFFERENTIAL CALCULUS – anagram of CUL DE SAC FALLEN FRUIT
14. BLOOD DONOR – B (born) + ODD (funny) in LOON (fool) + OR (gold)
19. SCARCE – SCAR (trauma) + CE (church)
20. FORMAT – FOR (for) + MAT (surface protection)
23. LAIR – L (line) + AIR (expose)