Animal that's not terribly content (5)
I believe the answer is:
'animal' is the definition.
(I've seen this before)
'that's not terribly content' is the wordplay.
'content' says the answer is hidden in the clue (I've seen 'contents of' mean this).
'OTTER' is hidden within '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" .)