Maybe not a weight? (3)
I believe the answer is:
ton
'a weight?' is the definition.
(ton is a kind of weight)
'maybe not' is the wordplay.
'maybe' is an anagram indicator.
'not' anagrammed gives 'TON'.
(Other definitions for ton that I've seen before include "High-speed" , "Unit of weight equivalent to 2240 pounds" , "Style" , "Fashionable" , "Imperial unit of weight" .)