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 viscid substance", "Kind of rat for sailor", "Bitumen", "Dark viscous flammable substance", "Dark flammable liquid", "Product of burning tobacco", "Pitch; sailor", "Dark liquid used on roads", "Viscous bituminous substance" and "Sailor; bitumen".)