Seaman can be this clever (4)
I believe the answer is:
able
I believe this is a triple definition!
'seaman' is the first definition.
(an able seaman is a type of seaman)
'can' is the second definition.
(I've seen this in other clues)
'be' acts as a link.
'this clever' is the third definition.
(I've seen this before)
(Other definitions for able that I've seen before include "Having considerable skill" , "Of high proficiency" , "Having the ability to do something" , "With the required capacities" , "Competent, having ability" .)