Backstage area (9)
(Other definitions for greenroom that I've seen before include "where there's relaxed entertainment" , "Backstage resting place for actors" , "Kind of lounge in theatre for off-stage actors" , "Place for actors to relax with their friends" , "Colourful rest area in theatre for actors" .)
'backstage area' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for green room that I've seen before include "Where actors can relax in the theatre" , "Place for actors to relax" , "Part of theatre" , "theatre? Only a bit of it" , "Here's the rest of the cast" .)