Selfishness, say, over one's millions (6)
I believe the answer is:
egoism
'selfishness' is the definition.
(I've seen this before)
'say over one's millions' is the wordplay.
'say' becomes 'eg' (both can mean for example).
'over' becomes 'o' (cricket abbreviation).
'one's' becomes 'i's' (Roman numeral).
'millions' becomes 'm' (abbreviation for millions).
'eg'+'o'+'is'+'m'='EGOISM'
(Other definitions for egoism that I've seen before include "sense of superiority" , "self-centred doctrine" , "Exaggerated sense of one's own importance" , "Self interest" , "display of selfishness" .)