Horse and colt beset by flipping flatus (4)
I believe the answer is:
scag
'horse' is the definition.
(synonyms)
'colt beset by flipping flatus' is the wordplay.
'colt' becomes 'c' (horseracing abbreviation).
'beset by' indicates putting letters inside ('beset' could historically mean 'cover').
'flipping' says the letters should be written backwards.
'flatus' becomes 'gas' (synonyms).
'gas' backwards is 'sag'.
'c' going within 'sag' is 'SCAG'.
'and' acts as a link.
(Other definitions for scag that I've seen before include "heroin" , "Sugar" .)