Creature in glasses (3)
I believe the answer is:
ass
'creature' is the definition.
(term for a donkey)
'in glasses' is the wordplay.
'in' says the answer is hidden in the clue.
'ASS' is hidden within 'glasses'.
(Other definitions for ass that I've seen before include "Clot" , "Silly fool or beast of burden" , "Trump's behind" , "mug" , "A donkey or an idiot" .)