Heavy weight cask picked up (3)
I believe the answer is:
ton
'heavy weight' is the definition.
(a ton weighs a lot)
'cask picked up' is the wordplay.
'cask' becomes 'tun' (tun is a kind of cask).
'picked up' shows a homophone (sound like).
'tun' sounds like 'TON'.
(Other definitions for ton that I've seen before include "A slang 100 mph" , "Speed" , "20cwt" , "Two thousand - two hundred and forty pounds" , "High-speed" .)