Fraction of a ruble (6)
I believe the answer is:
kopeck
'fraction of a ruble' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for kopeck that I've seen before include "Russian copper" , "One-hundredth of a rouble" , "One hundred of these make one rouble" , "little Russian money" , "Russian coin" .)