Fractional monetary unit (4)
I believe the answer is:
cent
'fractional monetary unit' is the definition.
(cent is a kind of fractional monetary unit)
This is the entire clue.
(Other definitions for cent that I've seen before include "Tiny coin" , "One-hundredth of the value of the basic unit" , "One 100th of a dollar" , "One-hundredth of a currency" , "One tenth of a dime" .)