Fish: southern constellation (5)

I believe the answer is:

sargo

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This clue could be a double definition.

'fish' is the first definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?

'southern constellation' is the second definition.
The definition suggests a singular noun which matches the answer.

Can you help me to learn more?

I've seen this clue in The Sunday Times and The Times.

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