Great wealth: chap gathering millions upon millions, nothing more (6)

I believe the answer is:

mammon

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'great wealth' is the definition.
'mammon' can be an answer for 'wealth' (term for the sins and temptations of wealth). I'm not sure about the 'great' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mammon that I've seen before include "Wealth (worshipped)" , "Greed personified" , "God of material wealth" , "money" , "Bible 'You cannot serve God and . . . . . .'" .)

I've seen this clue in The Independent.

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