Actors in crowd scenes (6)
I believe the answer is:
extras
'actors in crowd scenes' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for extras that I've seen before include "Add-ons" , "walk-on parts" , "Additional items surplus to requirements, spares" , "Bit players in film" , "Bit players, actors without dialogue" .)