An occasional brief for our spies (4)

I believe the answer is:

asio

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

'our spies' is the definition.
I can't judge whether this defines the answer.

'an occasional brief' is the wordplay.
'ASIO' is hidden within the letters.
However, I am uncertain how the hidden word is indicated.

'for' acts as a link.

Can you help me to learn more?

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

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