Heavyweight beer cask, by the sound of it (3)
I believe the answer is:
ton
'heavyweight' is the definition.
(I've seen this before)
'beer cask by the sound of it' is the wordplay.
'beer cask' becomes 'tun' (I've seen this before).
'by the sound of it' shows a homophone (sound like).
'tun' is a homophone of 'TON'.
(Other definitions for ton that I've seen before include "1000 kilos" , "A slang 100 mph" , "2240lbs" , "Two thousand, two hundred and forty lbs avoirdupois" , "Fashionable" .)