Warhorse once — source of continuing power (7)
I believe the answer is:
charger
'warhorse' is the definition.
(charger is a kind of warhorse)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for charger that I've seen before include "One demanding payment" , "Horse - plate" , "One may provide juice" , "He doesn't mean it to be a gift" , "Warhorse - electrical device - plate" .)