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' (term for an old horse).
'aboard' means one lot of letters goes inside another.
'ship?' becomes 'SS' (prefix in ship names eg SS Great Britain).
'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" .)