The seaman's clever art (3)
I believe the answer is:
tar
'the seaman's' is the definition.
('tar' can be a synonym of 'seaman')
'clever art' is the wordplay.
'clever' is an anagram indicator.
'art' anagrammed gives 'TAR'.
(Other definitions for tar that I've seen before include "one may need to pay on deck" , "Mariner, old salt" , "Product of coal" , "Thick, black and sticky liquid" , "Product of burning tobacco" .)