To us, it can be useful for keeping going (4)
I believe the answer is:
fuel
'keeping going' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'to us it can be useful' is the wordplay.
I cannot really understand how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
The remaining letters 'ful' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'for' is the link.
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?" .)