What's left on, the electricity supply? (7)
I believe the answer is:
remains
'what's left' is the definition.
(what remains or is left behind)
'on the electricity supply?' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'the electricity supply?' becomes 'mains' (I've seen this before).
're'+'mains'='REMAINS'
(Other definitions for remains that I've seen before include "surviving old parts" , "dead body the result?" , "What's left (maybe of a body)" , "Leftovers" , "Body after death" .)