Beast, cad — no leader (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')
'cad no leader' is the wordplay.
'cad' becomes 'rotter' (I've seen this in another clue).
'no leader' means to remove the first letter.
'rotter' with its first letter removed is 'OTTER'.
(Other definitions for otter that I've seen before include "eg Tarka" , "Carnivorous aquatic mammal" , "Fur possibly" , "carnivore is concerned" , "Fish-eating member of weasel family" .)