Participating in film, obscure crowds (4)
I believe the answer is:
mobs
'crowds' is the definition.
(I know this)
'participating in film obscure' is the wordplay.
'participating in' indicates the answer is hidden within the clue.
'MOBS' can be found hidden inside 'film obscure'.
(Other definitions for mobs that I've seen before include "Disorderly crowds of people" , "Unruly crowds" , "riotous assemblies" .)