Scoundrel beheaded animal (5)
I believe the answer is:
otter
'animal' is the definition.
(thesaurus)
'scoundrel beheaded' is the wordplay.
'scoundrel' becomes 'rotter' (I've seen this before).
'beheaded' means to remove the first letter.
'rotter' with its initial letter taken off is 'OTTER'.
(Other definitions for otter that I've seen before include "carnivore is concerned" , "Aquatic creature" , "Water animal with webbed fee, brown fur and flat tail" , "Fur possibly" , "furry creature" .)