Beast is cad dismissed at the outset (5)
I believe the answer is:
otter
'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" .)