Hitch to small horse (4)
I believe the answer is:
snag
'hitch' is the definition.
(I know that snag is a more specific form of the action hitch)
'small horse' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'horse' becomes 'nag' (term for an old horse).
's'+'nag'='SNAG'
'to' acts as a link.
(Other definitions for snag that I've seen before include "Complication, drawback" , "Hidden obstacle or catch" , "Hitch, unexpected problem" , "ladder" , "Hitch, obstacle" .)