Number about to be trapped by beast -- not the type gentlemen prefer? (8)
I believe the answer is:
brunette
'not the type gentlemen prefer?' is the definition.
('gentlemen prefer blondes')
'number about to be trapped by beast' is the wordplay.
'number' becomes 'ten' (ten is a kind of number).
'about' says the letters should be written backwards.
'to be trapped by' means one lot of letters goes inside another.
'beast' becomes 'brute' (synonyms).
'ten' written backwards gives 'net'.
'net' put inside 'brute' is 'BRUNETTE'.
(Other definitions for brunette that I've seen before include "Possessor of dark brown 22 ac" , "Dark-haired woman" , "Hair colour" , "Dark lady" , "Brown-haired" .)