Greed of girl over cereal (7)
I believe the answer is:
avarice
'greed' is the definition.
(avarice is a kind of greed)
'girl over cereal' is the wordplay.
'girl' becomes 'ava'.
'over' means one lot of letters go next to another.
'cereal' becomes 'rice' (rice is a kind of cereal).
'ava'+'rice'='AVARICE'
'of' is the link.
(Other definitions for avarice that I've seen before include "excessive desire for wealth" , "Cupidity" , "Excessive greed for wealth" , "Greed - caviare (anag)" , "Wanting to be rich" .)