The French one is not fat (4)
I believe the answer is:
lean
'not fat' is the definition.
('lean' is the opposite to 'fat')
'the french one' is the wordplay.
'the french' becomes 'le'.
'one' becomes 'an' (I've seen this in other clues**).
'le'+'an'='LEAN'
'is' acts as a link.
(Other definitions for lean that I've seen before include "Incline, bend" , "Slope; thin" , "Unprofitable" , "Inclination" , "Slant" .)