Essential part of electricity bill (6)
I believe the answer is:
charge
'bill' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for charge that I've seen before include "rate" , "Offensive - fee" , "Attack - require payment" , "person in custody" , "Lord Cardigan's order to his brigade" .)