Waylaying accountant to go over expense of signage used now and then (9)

'over expense of signage used now and then' is the definition.
I can't judge whether this definition defines the answer.

'waylaying accountant to' is the wordplay.
'waylaying' means one lot of letters goes inside another (inserted letters are held or waylaid).
'accountant' becomes 'cost accountant' (cost accountant is a kind of accountant).
'to' becomes 'acing' (I can't explain this - if you can you should give a lot more credence to this answer).
'cost' inserted into 'acing' is 'ACCOSTING'.

'go' is the link.

Can you help me to learn more?