Sailor finds the French skilful (4)
I believe the answer is:
'skilful' is the definition.
(I've seen this before)
'sailor finds the french' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'finds' means one lot of letters go next to another.
'the french' becomes 'le' ('the' in French).
(Other definitions for able that I've seen before include "- -bodied; - seaman" , "accomplished" , "Skilled" , "Capable seaman" , "Competent as, say, sailor" .)