A seaman can be so skilled (4)
I believe the answer is:
able
I believe this clue is a double definition.
'a seaman' is the first definition.
(an able seaman is a type of seaman)
'skilled' is the second definition.
(I've seen this in another clue)
'can be so' acts as a link.
(Other definitions for able that I've seen before include "accomplished" , "Capable or proficient" , "Having the skills and qualifications required" , "Fully competent" , "Skilled" .)