Incline to be slender (4)
I believe the answer is:
lean
I believe this clue is a triple definition!
'incline' is the first definition.
('lean' can be a synonym of 'incline')
'to be' is the second definition.
(leaning is a kind of being)
'slender' is the third definition.
('lean' can be similar in meaning to 'slender')
(Other definitions for lean that I've seen before include "Thin; rest (against)" , "Incline; without fat" , "Meagre" , "Without excess flesh" , "Of meat, with little or no fat" .)