Hitch second horse (4)
I believe the answer is:
snag
'hitch' is the definition.
(snagging is a kind of hitching)
'second horse' is the wordplay.
'second' becomes 's' (common abbreviation).
'horse' becomes 'nag' (nag is a kind of horse).
's'+'nag'='SNAG'
(Other definitions for snag that I've seen before include "Tear - catch" , "Minor problem" , "Complication, drawback" , "Jagged projection" , "Stumbling block" .)