Animal that's not terribly content (5)
I believe the answer is:
otter
'animal' is the definition.
(I know that otter is a type of animal)
'that's not terribly content' is the wordplay.
'content' indicates a hidden word (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 "Something furry" , "Water-loving animal" , "Fish-eating water animal with brown fur and flat tail" , "River creature" , "Playful aquatic animal of 'Tarka' fame" .)