Crustacean sometimes caught in a row (4)

I believe the answer is:

crab

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'crustacean' is the definition.
(I know that crab is a type of decapod)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for crab that I've seen before include "The constellation Cancer" , "Marine crustacean (seen on 1 across?)" , "Seafood (that can be caught by a rower?)" , "A crustacean; type of apple" , "Food from the sea" .)

I've seen this clue in The Irish News.

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