Plain cask difficult to lift, though not hot (6)
I believe the answer is:
tundra
'plain' is the definition.
(I know that tundra is a type of plain)
'cask difficult to lift though not hot' is the wordplay.
I cannot quite see how this works, but
'cask' could be 'tun' (tun is a kind of cask) and 'tun' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tundra that I've seen before include "Vast level Arctic region with frozen subsoil" , "Vast Artic region with frozen sub-soil" , "Northern permafrost region" , "Sub-polar treeless zone" , "Cold northern region" .)