Crowd in demo, belligerent (3)
I believe the answer is:
mob
'crowd' is the definition.
(mob can mean a crowd of people)
'in demo belligerent' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MOB' is hidden within 'demo belligerent'.
(Other definitions for mob that I've seen before include "press round" , "Ugly crowd" , "Rabble" , "Kind of cap" , "Horde" .)