Thinner, with less fat (6)
I believe the answer is:
leaner
I believe this clue is a double definition.
'thinner' is the first definition.
('lean' can be a synonym of 'thin')
'with less fat' is the second definition.
(I've seen this in another clue)
(Other definitions for leaner that I've seen before include "less bulky" , "less productive" , "Thinner, with less fat" , "less corpulent?" , "One with an inclination" .)