Small sum (4)
I believe the answer is:
mite
'small sum' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mite that I've seen before include "The widow's small offering" , "Minute creature" , "Tiny arachnid, person or amount of money" , "Minute arachnid" , "Microscopic creature" .)