Not over a hundred (3)
I believe the answer is:
ton
'a hundred' is the definition.
(I've seen this before)
'not over' is the wordplay.
'over' shows that the letters should be reversed in order.
'not' written backwards gives 'TON'.
(Other definitions for ton that I've seen before include "Twenty cwt" , "Two thousand, two hundred and forty lbs avoirdupois" , "High-speed" , "Imperial unit of weight" , "2,240 lbs" .)