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 wordplay.
'a horse' becomes 'nag' (nag is a kind of horse).
'in' indicates putting letters inside.
'a ship' becomes 'ss'.
'nag' placed inside 'ss' is 'SNAGS'.
(Other definitions for snags that I've seen before include "Hitches", "Problems", "Obstacles", "Drawbacks", "Unexpected obstacles" and "Unforeseen obstacles".)