Large cask said to weigh a lot (3)
I believe the answer is:
tun
'large cask' is the definition.
(type of container for alcohol)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tun that I've seen before include "One can hold wine" , "'Large cask, for beer perhaps (3)'" , "Large cask (holding 252 gallons)" , "Quite enough wine" , "Barrel holding many gallons" .)