The seaman’s cunning art (3)
I believe the answer is:
tar
'the seaman's' is the definition.
('tar' can be a synonym of 'seaman')
'cunning art' is the wordplay.
'cunning' indicates an anagram.
'art' is an anagram of 'TAR'.
(Other definitions for tar that I've seen before include "used by those who pay to work on deck" , "Flammable liquid; sailor" , "Product of smoking cigarettes" , "Jack < to blacken" , "prepare for feathers?" .)