Incompetent sailor between French articles (6)
I believe the answer is:
unable
'incompetent' is the definition.
(I've seen this before)
'sailor between french articles' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'between' means one lot of letters goes inside another.
'french articles' becomes 'un le' ('un' and 'le' are articles in French).
'ab' placed within 'unle' is 'UNABLE'.
(Other definitions for unable that I've seen before include "Lacking the necessary skills" , "Impotent" , "Not having the necessary means or skill or know-how" , "Lacking power (to do something)" , "Not having sufficient power" .)