Seems a bit crabby perhaps, when rook is used to intercept two other chess pieces (4,5)

I believe the answer is:

king prawn

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'intercept two other chess pieces' is the definition.
The definition suggests a singular noun which matches the answer.

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

Can you help me to learn more?

(Other definitions for king prawn that I've seen before include "Piece of seafood" , "(Regal?) seafood" , "crustacean" .)

I've seen this clue in The Irish Times.

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