Round lump sum of money in ancient Greece (6)

I believe the answer is:

obolus

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'money in ancient greece' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'round lump sum' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'lump sum' becomes 'bolus' (I can't justify this - if you can you should believe this answer much more).
'o'+'bolus'='OBOLUS'

'of' is the link.

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