Sack clerk (7)
I believe the answer is:
cashier
This clue is a double definition.
'sack' is the first definition.
(I've seen this before)
'clerk' is the second definition.
(eg a bank clerk)
(Other definitions for cashier that I've seen before include "Remove from service" , "Bank teller" , "Teller - dismiss in disgrace" , "One handles the money" , "Employee who takes in or gives out money" .)