Refill with petrol regularly, working pump evenly (3-2)

I believe the answer is:

top-up

I'm a little stuck... Click here to teach me more about this clue!

But I can't explain it at all! Could you help me understand?

(Other definitions for top-up that I've seen before include "Replenished drink" , "Supplement" , "Extra addition" , "Additional amount to restore mobile phone credit, for example" , "Amount required to restore something's level" .)

I've seen this clue in the Independent.

Want a hint initially instead of a full solution? Install my app