Aquatic creature skunk beheaded (5)
I believe the answer is:
otter
'aquatic creature' is the definition.
(animal that lives by water)
'skunk beheaded' is the wordplay.
'skunk' becomes 'rotter' (synonyms).
'beheaded' means to remove the first letter (the head of the word is removed).
'rotter' with its initial letter taken off is 'OTTER'.
(Other definitions for otter that I've seen before include "Torte (anag)" , "furry animal" , "Mammal, 'Tarka'" , "Tarka, for one" , "River animal" .)