Top up fee (6)
I believe the answer is:
charge
This clue might be a double definition.
'top up' is the first definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'fee' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for charge that I've seen before include "person in custody" , "Price for a service" , "responsibility" , "Accusation (in court)" , "How much" .)