A Parisian that's fit, but impotent? (6)
I believe the answer is:
unable
'impotent?' is the definition.
('unable' can be a synonym of 'impotent')
'a parisian that's fit' is the wordplay.
'a parisian' becomes 'un' ('a' in French).
'that's fit' becomes 'able' (similar in meaning).
'un'+'able'='UNABLE'
'but' acts as a link.
(Other definitions for unable that I've seen before include "Incapable" , "Having insufficient strength or ability (to do something)" , "Incompetent sort of nebula" , "Impotent" , "Lacking the required capacity" .)