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 cannot tell whether it can be defined by this definition.
'be paid for' is the wordplay.
I cannot quite see 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 clue (or it may be nonsense).
'to' is the 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" .)