Not fat (4)
'not fat' is the definition.
(I know this)
This is all the clue.
(Other definitions for thin that I've seen before include "Slender, narrow" , "(Of smile) weak, forced" , "Skinny" , "Carrying little fat" , "With little thickness" .)
(Other definitions for lean that I've seen before include "Sir David - - (film director)" , "'Tend, incline (4)'" , "Tend towards being thin" , "Slight - incline" , "Of meat, with little or no fat" .)