Clever sailor is attached to the French (4)
I believe the answer is:
able
'clever' is the definition.
(thesaurus)
'sailor is attached to the french' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'is attached to' is a charade indicator (letters next to each other).
'the french' becomes 'le'.
'ab'+'le'='ABLE'
(Other definitions for able that I've seen before include "Having great proficiency or intelligence" , "Skilled" , "Fit as a seaman" , "Competent and fit, like sailor" , "See 10" .)