Underground space (6)
I believe the answer is:
cellar
'underground space' is the definition.
(I know that cellar is a type of underground space)
This is the entire clue.
(Other definitions for cellar that I've seen before include "Underground area" , "Storage space, for wine perhaps" , "Underground room - stock of bottled wines" , "Storage area for the wine" , "Underground store room" .)