Crossword Genius

Beast in river more fiery for Cockneys (5)

Ross

I believe the answer is:

otter

I'm a little stuck... Click here to teach me more about this clue!

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

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for otter that I've seen before include "Fish-eater of the weasel family" , "Aquatic beast" , "Fish-eating water animal with brown fur and flat tail" , "Fish-eating mammal" , "Good swimmer" .)

I've seen this clue in The Sun.
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