Heading off rat, aquatic animal (5)
I believe the answer is:
otter
'aquatic animal' is the definition.
(animal that lives by water)
'heading off rat' is the wordplay.
'heading off' means to remove the first letter.
'rat' becomes 'rotter' (synonyms).
'rotter' with its initial letter taken away is 'OTTER'.
(Other definitions for otter that I've seen before include "Tarka, for one" , "Mammal, 'Tarka'" , "river in Honiton" , "Semi-aquatic mammal" , "Mine is made safe by this" .)