Room still lacking support walls, we hear (5)
I believe the answer is:
attic
'room' is the definition.
(type of room)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for attic that I've seen before include "The highest room" , "Top apartment" , "Greek - upper room" , "Room inside a building's roof space" , "Athenian - garret" .)