Beware a hollow space! (4)
I believe the answer is:
cave
I believe this is a double definition.
'beware' is the first definition.
(old-fashioned term meaning 'look out')
'a hollow space' is the second definition.
(underground space)
(Other definitions for cave that I've seen before include "Watch out" , "Warning - grotto" , "hole" , "Natural subterranean opening" , "Underground enclosure with surface access" .)