Be inclined to be slim (4)
I believe the answer is:
lean
I believe this clue is a triple definition!
'be inclined' is the first definition.
(synonyms)
'to be' is the second definition.
(leaning is a kind of being)
'slim' is the third definition.
(similar in meaning)
(Other definitions for lean that I've seen before include "Without excess flesh" , "Insufficient - incline" , "Containing little fat" , "Current update (colloq.)" , "List; thin" .)