Small fund for incidental expenses (5,4)
I believe the answer is:
petty cash
'small fund for incidental expenses' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for petty cash that I've seen before include "money kept for small expenses" , "Money kept in office for small payments - it's trivial" , "Small amounts of money set aside for insignificant payments" , "Small fund for minor expenses" , "Money for small day-to-day expenses" .)