Scoundrel taking head off freshwater mammal (5)
I believe the answer is:
otter
'freshwater mammal' is the definition.
(I've seen this before)
'scoundrel taking head off' is the wordplay.
'scoundrel' becomes 'rotter' (both can mean a nasty person).
'taking head off' means to remove the first letter.
'rotter' with its initial letter removed is 'OTTER'.
(Other definitions for otter that I've seen before include "Water creature" , "Fish-eating member of weasel family" , "Fur possibly" , "Sleek river mammal" , "fish-eater displayed" .)