Incline to be the reverse of fat (4)
I believe the answer is:
lean
This clue might be a double definition.
'incline' is the first definition.
(I know that incline can be written as lean)
'be the reverse of fat' is the second definition.
'lean' can be an answer for 'be' (leaning is a kind of being). I'm unsure of the 'the reverse of fat' bit.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for lean that I've seen before include "director in past" , "Thin; rest (against)" , "Incline; (of person) thin" , "Without excess flesh" , "Spare - incline" .)