Fit as a seaman? (4)
I believe the answer is:
able
I believe this is a double definition.
'fit' is the first definition.
(similar in meaning)
'a seaman?' is the second definition.
(an able seaman is a type of seaman)
'as' acts as a link.
(Other definitions for able that I've seen before include "Having the skills and qualifications required" , "Proficient; skilful" , "Competent and equipped" , "Bale out, being competent" , "See 10" .)