Not over weight (3)
I believe the answer is:
ton
'weight' is the definition.
(ton is a kind of weight)
'not over' is the wordplay.
'over' says the letters should be written in reverse.
'not' reversed gives '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" .)