The seaman's clever art (3)
I believe the answer is:
Here is my best explanation:
'the seaman's' is the definition.
('tar' can be a synonym of 'seaman')
'clever art' is the subsidiary indication.
'clever' indicates an anagram.
'art' anagrammed gives 'tar'.
(Other definitions for tar that I've seen before include "Dark flammable liquid", "Jack ..., British sailor", "It paves the way", "Informal sailor or road surfacing material", "Product of coal", "Dark viscous stuff", "Mariner, old salt", "Informal name for sailor", "Old salt or liquid from coal" and "Crewman".)