Very large pot used for boiling (8)
I believe the answer is:
cauldron
'very large pot used for boiling' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cauldron that I've seen before include "Place that's seething" , "Very large metal pot for boiling" , "A large deep pot for boiling things" , "Witches' pot" , "Witch's brewing pot" .)