Hitches small horses (5)
I believe the answer is:
snags
'hitches' is the definition.
(snagging is a kind of hitching)
'small horses' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'horses' becomes 'nags' (nag is a kind of horse).
's'+'nags'='SNAGS'
(Other definitions for snags that I've seen before include "Unforeseen obstacles" , "Unexpected obstacles" , "Drawbacks" , "Hitches" , "Problems" .)