Crossword Genius

Contents of many an ancient vessel (5)

Ross

I believe the answer is:

cargo

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'contents' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'many an ancient vessel' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'an ancient vessel' becomes 'argo' (I've seen this in another clue).
'c'+'argo'='CARGO'

'of' is the link.

Can you help me to learn more?

(Other definitions for cargo that I've seen before include "This taken on board" , "Freight, load" , "What gets carried" , "Goods carried by a large vehicle" , "Ship's load" .)

I've seen this clue in The Globe and Mail.
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