Hitch small horse (4)
I believe the answer is:
snag
'hitch' is the definition.
(I know that hitch can be written as snag)
'small horse' is the wordplay.
'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 "Hidden obstacle or catch" , "Unexpected drawback" , "Hidden drawback; tear" , "Hitch, obstacle" , "Tear - catch" .)