Animal engaged in eating nuts (3)
I believe the answer is:
gnu
'animal' is the definition.
(type of antelope)
'engaged in eating nuts' is the wordplay.
'engaged in' indicates a hidden word.
'GNU' is hidden in the letters of 'eating nuts'.
(Other definitions for gnu that I've seen before include "Literally wild beast" , "Creature" , "hitherto unknown, we hear" , "Gun to get antelope" , "Large heavy antelope" .)