Disadvantage of small horse (4)
I believe the answer is:
snag
'disadvantage' is the definition.
(I've seen this before)
'small horse' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'horse' becomes 'nag' (term for an old horse).
's'+'nag'='SNAG'
'of' is the link.
(Other definitions for snag that I've seen before include "Disadvantage - small hole in fabric" , "Sang about obstacle" , "Slight problem" , "Difficulty; catch" , "A hidden obstruction" .)