Beast is cad dismissed at the outset (5)

I believe the answer is:

otter

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'beast' is the definition.
(I have seen 'Aquatic beast' mean 'otter' so perhaps 'beast' could also mean 'otter')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for otter that I've seen before include "Aquatic weasel-like mammal" , "Aquatic beast" , "Water creature" , "Something furry" , "Semi-aquatic creature" .)

I've seen this clue in The Sun.

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