How the interrogator got inside? (6,9)

I believe the answer is:

gained admission

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'inside?' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?

'how the interrogator got' is the wordplay.
I cannot really understand how this works, but
'got' could be 'gained' (gaining is a kind of getting) and 'gained' is found within the answer.
The remaining letters 'admission' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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