Beast that's not terribly content (5)
I believe the answer is:
otter
'beast' is the definition.
(I have seen 'Aquatic beast' mean 'otter' so perhaps 'beast' could also mean 'otter')
'that's not terribly content' is the wordplay.
'content' indicates the answer is hidden within the clue (letters contained within the clue).
'OTTER' is hidden in the letters of 'that not terribly'.
(Other definitions for otter that I've seen before include "Fur possibly" , "eg Tarka" , "Fish-eater of the weasel family" , "river in Honiton" , "Playful aquatic animal of 'Tarka' fame" .)