Basic oven supplied covers these assemblies (6)
I believe the answer is:
covens
'these assemblies' is the definition.
(coven is a kind of assembly)
'basic oven supplied covers' is the wordplay.
'covers' indicates a hidden word (I've seen 'covered' mean this (the answer is covered up by the clue)).
'COVENS' is hidden within 'basic oven supplied'.
(Other definitions for covens that I've seen before include "Gangs of witches" , "Assemblies of witches" , "Groups or witches" , "Gatherings of witches" .)