Playful aquatic mammal (5)
I believe the answer is:
otter
'playful aquatic mammal' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for otter that I've seen before include "bad-tempered little beast" , "River-dwelling mammal" , "Good swimmer" , "Aquatic member of weasel family" , "Mammal, 'Tarka'" .)