Sudden movement outside old room (6)
I believe the answer is:
lounge
'room' is the definition.
(I know that lounge is a type of room)
'sudden movement outside old' is the wordplay.
'sudden movement' becomes 'lunge' (I've seen this in another clue).
'outside' means one lot of letters goes inside another (some letters go outside others).
'old' becomes 'o' (abbreviation).
'lunge' going around 'o' is 'LOUNGE'.
(Other definitions for lounge that I've seen before include "Sprawl" , "Room for relaxing" , "Living area" , "Relax in sitting room?" , "Waste time -- glue on (anag)" .)