A cad - a right beast! (6)
I believe the answer is:
rotter
'a cad' is the definition.
(I've seen this in another clue)
'a right beast' is the wordplay.
I cannot really see how this works, but
'right' could be 'r' (common abbreviation) and 'r' is found within the answer.
'beast' could be 'otter' (I have seen 'Aquatic beast' mean 'otter' so perhaps 'beast' could also mean 'otter') and 'otter' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rotter that I've seen before include "baddie" , "Absolute bounder" , "Something going off" , "One decomposing" , "rat" .)