Top up, replenish (6)
I believe the answer is:
refill
This clue is a double definition.
'top up' is the first definition.
'replenish' is the second definition.
(I know that replenish can be written as refill)
(Other definitions for refill that I've seen before include "Solution for dried-up writer" , "Replace contents" , "Charge again" , "Top up glass, replenish" , "Top up glass or petrol tank" .)