Sailor makes difficult tack in a vessel (4,3)
I believe the answer is:
jack tar
'sailor' is the definition.
(jack-tar is a kind of sailor)
'difficult tack in a vessel' is the wordplay.
'difficult' indicates an anagram.
'in' indicates putting letters inside.
'a vessel' becomes 'jar' (jar is a kind of vessel**).
'tack' with letters rearranged gives 'ackt'.
'ackt' placed within 'jar' is 'JACK-TAR'.
'makes' acts as a link.
(Other definitions for jack tar that I've seen before include "Serviceman" , "One singing shanty" , "(Jolly) sailor" .)