Beast in river more fiery for Cockneys (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 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" .)