Nothing about being slim (4)
I believe the answer is:
thin
'being slim' is the definition.
('thin' can be similar in meaning to 'slim')
'nothing about' is the wordplay.
'about' indicates the answer is hidden within the clue (the clue goes around or about the answer).
'THIN' is hidden within 'nothing'.
(Other definitions for thin that I've seen before include "Skinny" , "Fine" , "Slim - watery" , "Watered down" , "Scrawny" .)