Swap a cold urn for a big pot (8)
I believe the answer is:
cauldron
'big pot' is the definition.
'cauldron' can be an answer for 'pot' (I've seen this before). I'm not certain of the 'big' bit.
'swap a cold urn' is the wordplay.
'swap' indicates an anagram (I've seen 'swaps' mean this).
'a'+'cold'+'urn'='acoldurn'
'acoldurn' anagrammed gives 'CAULDRON'.
'for a' is the link.
Can you help me to learn more?
(Other definitions for cauldron that I've seen before include "Large cooking pot" , "Witches' vessel" , "A large deep pot for boiling things" , "Place that's seething" , "Very large metal pot for boiling" .)