Hitch the small horse (4)
I believe the answer is:
snag
'hitch' is the definition.
(synonyms)
'the small horse' is the wordplay.
'the small' becomes 's' (abbreviation - e.g. clothes size).
'horse' becomes 'nag' (term for an old horse).
's'+'nag'='SNAG'
(Other definitions for snag that I've seen before include "Glitch" , "Hidden obstacle or drawback" , "A hidden or unexpected obstacle" , "Hidden obstacle or catch" , "ladder" .)