Cruel fluctuation in money (5)
I believe the answer is:
lucre
'money' is the definition.
(I know that lucre is a type of money)
'cruel fluctuation' is the wordplay.
'fluctuation' indicates an anagram.
'cruel' is an anagram of 'LUCRE'.
'in' acts as a link.
(Other definitions for lucre that I've seen before include "Cash" , "Possession of the rich" , "benefits from crime" , "Scornful word for money that may be 1 Across" , "monetary gain" .)