The snag about horses (4)
I believe the answer is:
nags
'horses' is the definition.
(nag is a kind of horse)
'the snag about' is the wordplay.
'about' indicates anagramming the letters.
'snag' with letters rearranged gives 'NAGS'.
(Other definitions for nags that I've seen before include "Worries; horses" , "Pesters" , "Constantly reproaches and complains" , "Scolds continually" , "Scolds - horses" .)