To us, it can be useful for keeping going (4)
I believe the answer is:
fuel
'going' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for fuel that I've seen before include "Gas, oil or petrol, for instance" , "Eg, coal or oil" , "Burned source of energy" , "Substance burned to source power" , "Coal or gas?" .)