Not using small weights (4)
I believe the answer is:
tons
'weights' is the definition.
(ton is a kind of weight)
'not using small' is the wordplay.
'using' indicates anagramming the letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'not' anagrammed gives 'ton'.
'ton'+'s'='TONS'
(Other definitions for tons that I've seen before include "Loads (sl.)" , "Each is equivalent to 2240 pounds" , "A lot" , "Lots" .)