Because it's waiting to be paid for (2,7)
I believe the answer is:
on account
'because it's waiting' is the definition.
I know nothing about this answer so I can't tell whether this works.
'be paid for' is the wordplay.
I cannot quite understand how this works, but
'be' could be 'count' (counting is a kind of being) and 'count' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for on account that I've seen before include "By deferred payment" , "As partial payment with the rest to come later" , "Involving part payment" , "To be paid" , "an instalment" .)