Rescue vessel called to help a backward region in Greece once (7)

I believe the answer is:

arcadia

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

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for arcadia that I've seen before include "pastoral country" , "Idealised rural region" , "Idealised country region" , "rural idyll" , "pastoral paradise" .)

I've seen this clue in The Telegraph.

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