Animal in glasses (3)
I believe the answer is:
ass
'animal' is the definition.
(term for a donkey)
'in glasses' is the wordplay.
'in' says the answer is hidden in the clue.
'ASS' is hidden in the letters of 'glasses'.
(Other definitions for ass that I've seen before include "Stupid fellow" , "Animal; fool" , "Donkey or foolish person" , "Braying creature" , "Blockhead" .)