Animal's extreme idleness (5)
I believe the answer is:
sloth
This clue is a double definition.
'animal's' is the first definition.
(sloth is a kind of animal)
'extreme idleness' is the second definition.
(sloth can mean idleness)
(Other definitions for sloth that I've seen before include "Sluggish creature" , "Inertia" , "Sluggishness" , "Idleness" , "one of a deadly collection" .)