Not going back a great deal (3)
I believe the answer is:
ton
'a great deal' is the definition.
(both can mean a large amount)
'not going back' is the wordplay.
'going back' says the letters should be written backwards.
'not' back-to-front is 'TON'.
(Other definitions for ton that I've seen before include "Two thousand, two hundred and forty lb" , "Fashionable" , "20cwt" , "100 mph, runs, etc" , "Twenty cwt" .)