Reverse old carriage out of sight of the house (9)
I believe the answer is:
backstage
'out of sight of the house' is the definition.
(the house can mean a theatrical audience)
'reverse old carriage' is the wordplay.
'reverse' becomes 'back' (I've seen this before).
'old carriage' becomes 'stage' (stagecoach).
'back'+'stage'='BACKSTAGE'
(Other definitions for backstage that I've seen before include "Behind the scenes (theatrically)" , "away from public view" , "Wings etc" , "dressing room area" , "out of the limelight" .)