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' says the answer is hidden in the clue (letters contained within the clue).
'OTTER' can be found hidden inside '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" .)