Spies set up models, filling top floor room (5)
I believe the answer is:
attic
'top floor room' is the definition.
'spies set up models filling' is the wordplay.
'spies' becomes 'cia'.
'set up' is a reversal indicator.
'models' becomes 'tt' (I can't justify this - if you can you should believe this answer much more).
'filling' means one lot of letters goes inside another.
'cia' back-to-front is 'aic'.
'aic' placed around 'tt' is 'ATTIC'.
(Other definitions for attic that I've seen before include "High level of" , "Upper chamber" , "Room at the top of a house" , "Dusty room" , "Small room at the top" .)