There's a police officer in the room (5)
I believe the answer is:
scope
'room' is the definition.
(as in scope or room for manoeuvre)
'there's a police officer in' is the wordplay.
'there's a' becomes 'se' (I can't justify this - if you can you should give a lot more credence to this answer).
'police officer' becomes 'cop' (cop is a kind of police officer).
'in' means one lot of letters goes inside another.
'se' going around 'cop' is 'SCOPE'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for scope that I've seen before include "Ambit, compass" , "Extent of subject matter" , "Room" , "Extent or reach" , "Field" .)