Difficulties getting a horse aboard ship? (5)
I believe the answer is:
snags
'difficulties' is the definition.
(snag is a kind of difficulty)
'a horse aboard ship?' is the wordplay.
'a horse' becomes 'nag' (nag is a kind of horse).
'aboard' means one lot of letters goes inside another.
'ship?' becomes 'ss'.
'nag' inserted into 'ss' is 'SNAGS'.
'getting' is the link.
(Other definitions for snags that I've seen before include "Problems" , "Unexpected obstacles" , "Unforeseen obstacles" , "Hitches" , "Drawbacks" .)