Rat not initially an aquatic creature (5)
I believe the answer is:
otter
'an aquatic creature' is the definition.
(I've seen this before)
'rat not initially' is the wordplay.
'rat' becomes 'rotter' (synonyms).
'not initially' means to remove the first letter.
'rotter' with its first letter taken away is 'OTTER'.
(Other definitions for otter that I've seen before include "Tarka, for one" , "Sleek river mammal" , "Carnivorous aquatic mammal" , "Good swimmer" , "Playful aquatic animal of 'Tarka' fame" .)