Hitches a horse in a ship (5)
I believe the answer is:
Here is my best explanation:
'hitches' is the definition.
(snagging is a kind of hitching)
'a horse in a ship' is the subsidiary indication.
'a horse' becomes 'nag' (nag is a kind of horse).
'in' is an insertion indicator.
'a ship' becomes 'ss'.
'nag' placed within 'ss' is 'snags'.
(Other definitions for snags that I've seen before include "Obstacles", "Drawbacks", "Hitches", "Unexpected obstacles" and "Unforeseen obstacles".)