Because nothing can change the tally (2,7)
I believe the answer is:
on account
'because' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'nothing can change the tally' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'change' indicates anagramming the letters.
'the tally' becomes 'count' ('count' can be a synonym of 'tally').
'o'+'can'='ocan'
'ocan' with letters rearranged gives 'onac'.
'onac'+'count'='ON ACCOUNT'
Can you help me to learn more?
(Other definitions for on account that I've seen before include "As partial payment with the rest to come later" , "an instalment" , "Involving part payment" , "To be paid" , "By deferred payment" .)