Heavy weight? Perhaps not (3)
I believe the answer is:
ton
'heavy weight?' is the definition.
(a ton weighs a lot)
'perhaps not' is the wordplay.
'perhaps' indicates an anagram.
'not' is an anagram of 'TON'.
(Other definitions for ton that I've seen before include "1000 kilos" , "Unit of weight equivalent to 2240 pounds" , "Twenty cwt" , "2,240 lbs" , "100 mph, runs, etc" .)