Cooker isn't switched on (7)
I believe the answer is:
against
'on' is the definition.
(I've seen this before)
'cooker isn't switched' is the wordplay.
'cooker' becomes 'aga' (I've seen this before).
'switched' indicates anagramming the letters (I've seen 'switch' mean this).
'isnt' with letters rearranged gives 'inst'.
'aga'+'inst'='AGAINST'
(Other definitions for against that I've seen before include "In disagreement with" , "Opposing" , "Versus - in contact with" , "In opposition to" , "Con" .)