Nuts aren't about to worry animal (8)
I believe the answer is:
anteater
'animal' is the definition.
(anteater is a kind of animal)
'nuts aren't about to worry' is the wordplay.
'nuts' indicates an anagram (nuts can mean crazy).
'about' means one lot of letters goes inside another.
'to worry' becomes 'eat' (eating is a kind of worrying).
'arent' is an anagram of 'anter'.
'anter' enclosing 'eat' is 'ANTEATER'.
(Other definitions for anteater that I've seen before include "animal (this one?)" , "Rat-eaten? (anag)" , "Insectivorous mammal" , "It feeds on termites, etc" , "Termite muncher" .)