Be alert on board for crooks (6)

I believe the answer is:

sharks

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'crooks' is the definition.
The definition and answer can be both people as well as being plural nouns.
Maybe they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sharks that I've seen before include "Large carnivorous fish" , "Voracious sea fish" , "Feared fishes, may be basking or hammerhead" .)

I've seen this clue in The Globe and Mail.

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