Witch briefly keeping old Scottish pot for boiling (8)
I believe the answer is:
cauldron
'pot for boiling' is the definition.
'witch briefly keeping old scottish' is the wordplay.
'witch' becomes 'crone' (synonyms).
'briefly' means to remove the last letter.
'keeping' indicates putting letters inside.
'old scottish' becomes 'auld' (Scottish term for 'old').
'crone' with its final letter removed is 'cron'.
'cron' enclosing 'auld' is 'CAULDRON'.
(Other definitions for cauldron that I've seen before include "Large cooking pot" , "Place that's seething" , "Witches' pot" , "Very large pot used for boiling" , "A large deep pot for boiling things" .)