Key amount to pay (3)
I believe the answer is:
tab
This clue is a double definition.
'key' is the first definition.
(key on a computer keyboard)
'amount to pay' is the second definition.
(as in a bar tab)
(Other definitions for tab that I've seen before include "Till bill" , "Text indent key" , "Running bar bill" , "Check" , "Ring pull" .)