Not going back a lot (3)
I believe the answer is:
ton
'a lot' is the definition.
(I've seen this in another clue)
'not going back' is the wordplay.
'going back' shows that the letters should be reversed in order.
'not' reversed gives 'TON'.
(Other definitions for ton that I've seen before include "Weight - fashion (French)" , "Two thousand - two hundred and forty pounds" , "Century" , "Speed" , "Unit of weight equivalent to 2240 pounds" .)