Money transfer system is unknown possession of the Vatican (6)

I believe the answer is:

paypal

I'm a little stuck... Click here to teach me more about this clue!

'money transfer system' is the definition.
I can't tell whether this defines the answer.

'unknown possession of the vatican' is the wordplay.
I cannot quite see how this works, but
'unknown' could be 'y' (algebra) and 'y' is present in the answer.
'of the vatican' could be 'papal' (I've seen this in another clue) and 'papal' is located in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

'is' is the link.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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