Cruel fluctuation in profit (5)
I believe the answer is:
lucre
'profit' is the definition.
(synonyms)
'cruel fluctuation' is the wordplay.
'fluctuation' indicates anagramming the letters.
'cruel' anagrammed gives 'LUCRE'.
'in' acts as a link.
(Other definitions for lucre that I've seen before include "Money or wealth" , "Ulcer (anag)" , "profits" , "Archaic wealth, may be filthy" , "Ill-gotten riches" .)