It’s a kind of salmon, pal (4)

I believe the answer is:

chum

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'pal' is the definition.
(synonyms)

I don't understand how the remainder of the clue works.

(Other definitions for chum that I've seen before include "Good friend" , "Salmon" , "Close friend (colloq.)" , "Pal, mate" , "Informally, a close friend" .)

I've seen this clue in The Everyman puzzle in The Observer.

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