Getting a horse into a ship poses difficulties (5)
I believe the answer is:
snags
'poses difficulties' is the definition.
'snags' can be an answer for 'difficulties' (snag is a kind of difficulty). I am unsure of the 'poses' bit.
'getting a horse into a ship' is the wordplay.
'getting a horse' becomes 'nag' (I am not sure about this - if you are sure you should believe this answer much more).
'into' means one lot of letters goes inside another.
'a ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'nag' placed into 'ss' is 'SNAGS'.
Can you help me to learn more?
(Other definitions for snags that I've seen before include "Problems" , "Hitches" , "Drawbacks" , "Unexpected obstacles" , "Unforeseen obstacles" .)