Body concerned with electricity supply (7)
I believe the answer is:
remains
'body' is the definition.
(remains is a kind of body)
'concerned with electricity supply' is the wordplay.
'concerned with' becomes 're'.
'electricity supply' becomes 'mains' (I've seen this before).
're'+'mains'='REMAINS'
(Other definitions for remains that I've seen before include "Archaeological relics" , "Residue" , "Scraps" , "Body after death" , "What's left (maybe of a body)" .)