A Spanish agreement to ring Australian agency (4)

I believe the answer is:

asio

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'australian agency' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'a spanish agreement to ring' is the wordplay.
'spanish agreement' becomes 'si' (Spanish for yes).
'to' says to put letters next to each other.
'ring' becomes 'o' (O is the shape of a ring).
'a'+'si'+'o'='ASIO'

Can you help me to learn more?

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

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