Refill (5)
'refill' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for top up that I've seen before include "Add a bit more to" , "Extra amount" , "Provide a little more drink" , "Additional amount to restore mobile phone credit, for example" , "Amount for refilling" .)
(Another definition for topup that I've seen is " little more".)