Gang taking part in a demo, boldy (3)
I believe the answer is:
mob
'gang' is the definition.
(I know that gang can be written as mob)
'part in a demo boldy' is the wordplay.
'part in' says the answer is hidden in the clue.
'MOB' is hidden within 'demo boldy'.
'taking' is the link.
(Other definitions for mob that I've seen before include "Ugly crowd" , "Rabble" , "An undisciplined gang of people" , "press round" , "Mafiosi" .)