Perhaps not all that heavy? (3)
I believe the answer is:
ton
'all that heavy?' is the definition.
I can't tell whether this definition defines the answer.
'perhaps not' is the wordplay.
'perhaps' indicates anagramming the letters.
'not' is an anagram of 'TON'.
Can you help me to learn more?
(Other definitions for ton that I've seen before include "Heavy weight or 100mph" , "A slangy 100 mph - heavy" , "Two thousand, two hundred and forty lb" , "100 mph, runs, etc" , "20 cwt" .)