Old solver meets saint with cardinal (8)

I believe the answer is:

thousand

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'cardinal' is the definition.
(cardinal number)

'old solver meets saint with' is the wordplay.
'old solver meets' becomes 'thou' (I can't explain this - if you can you should give a lot more credence to this answer).
'saint' becomes 's'.
'with' becomes 'and'.
'thou'+'s'+'and'='THOUSAND'

Can you help me to learn more?

(Other definitions for thousand that I've seen before include "in MCC, but missing double century" , "M or K" , "Ten hundreds" , "Large number" , "quite a lot" .)

I've seen this clue in The Guardian.

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