Hitch horse to pole (4)
I believe the answer is:
snag
'hitch' is the definition.
(snagging is a kind of hitching)
'horse to pole' is the wordplay.
'horse' becomes 'nag' (nag is a kind of horse).
'to' means one lot of letters go next to another (I've seen this in other clues).
'pole' becomes 's'.
'nag' after 's' is 'SNAG'.
(Other definitions for snag that I've seen before include "Hidden drawback; tear" , "Disadvantage - small hole in fabric" , "Hidden obstacle or drawback" , "Minor hitch" , "Slight problem" .)