What may mark graves of saint and cardinal? (5)

I believe the answer is:

stone

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'what may mark graves' is the definition.
I can't judge whether this definition defines the answer.

'saint and cardinal?' is the wordplay.
'saint' becomes 'st' (abbreviation).
'and' says to put letters next to each other.
'cardinal?' becomes 'one' (a cardinal number).
'st'+'one'='STONE'

'of' is the link.

Can you help me to learn more?

(Other definitions for stone that I've seen before include "Kernel" , "Rock; cherry seed" , "See 21" , "Pebble" , "Rock; hard seed" .)

I've seen this clue in The Independent.

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