Provider of juice to warhorse (7)
I believe the answer is:
charger
This clue is a double definition.
'provider of juice' is the first definition.
('juice' can mean electrical charge)
'to warhorse' is the second definition.
(charger is a kind of warhorse)
(Other definitions for charger that I've seen before include "I expect payment" , "Cavalry horse - used on battery" , "Horse - plate" , "One demanding payment" , "He doesn't mean it to be a gift" .)