Not thin (3)
I believe the answer is:
fat
'not thin' is the definition.
('fat' is the opposite to 'thin')
This is all the clue.
(Other definitions for fat that I've seen before include "eg Lard" , "Obese, having spare tyre etc" , "and 24: Overpaid City type" , "Like 10 ac" , "Obese, overweight" .)