Chaps coming across to assist racehorse without a win (6)
I believe the answer is:
maiden
'racehorse without a win' is the definition.
(horseracing term)
'chaps coming across to assist' is the wordplay.
'chaps' becomes 'men' (chap means a man).
'coming across' is an insertion indicator.
'to assist' becomes 'aid' (synonyms).
'men' placed around 'aid' is 'MAIDEN'.
(Other definitions for maiden that I've seen before include "over, you won't score" , "No-score over" , "First ever public address" , "Unmarried girl of old" , "Dotty over" .)