What vessel is spoken of for spells? (10)
I believe the answer is:
witchcraft
'spoken of for spells?' is the definition.
I can't judge whether this definition defines the answer.
'what vessel' is the wordplay.
'what' becomes 'witch' (I can't justify this - if you can you should give a lot more credence to this answer).
'vessel' becomes 'craft' (synonyms).
'witch'+'craft'='WITCHCRAFT'
'is' is the link.
(Other definitions for witchcraft that I've seen before include "Frank Sinatra single" , "Supernatural power" , "[OBI], say" , "Sorcery skill" , "Magic" .)