What you might call rock salmon (4)

I believe the answer is:

huss

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

I cannot understand how the rest of the clue works.

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(Another definition for huss that I've seen is " Dogfish used as food".)

I've seen this clue in the Evening Standard.

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