Foist pet ermine upon gatekeeper? (7)

I believe the answer is:

stpeter

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'gatekeeper?' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'foist pet ermine' is the wordplay.
'STPETER' is hidden within the letters.
However, I'm uncertain how the hidden word is indicated.

'upon' is the link.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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