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