Not over weight (3)
I believe the answer is:
ton
'weight' is the definition.
(unit of weight)
'not over' is the wordplay.
'over' shows that the letters should be reversed in order.
'not' back-to-front is 'TON'.
(Other definitions for ton that I've seen before include "Two thousand, two hundred and forty lbs avoirdupois" , "Two thousand, two hundred and forty lb" , "High-speed" , "A slangy 100 mph - heavy" , "A slang 100 mph" .)