Thin; without fat (4)
I believe the answer is:
lean
I believe this clue is a double definition.
'thin' is the first definition.
(I know that thin can be written as lean)
'without fat' is the second definition.
(I've seen this before)
(Other definitions for lean that I've seen before include "Unproductive" , "With no fat; slope" , "Meagre" , "Current update (colloq.)" , "Tend towards being thin" .)