Incline to be thin (4)
I believe the answer is:
lean
I believe this clue is a triple definition!
'incline' is the first definition.
(I know that incline is a more specific form of lean)
'to be' is the second definition.
(leaning is a kind of being)
'thin' is the third definition.
(I know that thin can be written as lean)
(Other definitions for lean that I've seen before include "With no fat; slope" , "English film director" , "List; thin" , "Inclination" , "Insufficient - incline" .)