Creature of masses (3)
I believe the answer is:
ass
'creature' is the definition.
(term for a donkey)
'of masses' is the wordplay.
'of' indicates a hidden word.
'ASS' is hidden within 'masses'.
(Other definitions for ass that I've seen before include "Clinton's behind" , "Small hoofed animal" , "Stupid person or American bum" , "Long-eared animal" , "Idiotic animal" .)