Guard hollow chamber (4)
I believe the answer is:
cave
This clue could be a double definition.
'guard hollow' is the first definition.
'cave' can be an answer for 'hollow' (I've seen this in another clue). I'm unsure of the 'guard' bit.
'chamber' is the second definition.
(I have seen 'Underground chamber' mean 'cave' so perhaps 'chamber' could also mean 'cave')
Can you help me to learn more?
(Other definitions for cave that I've seen before include "Natural underground space" , "Troglodyte's home" , "Beware" , "Natural subterranean opening" , "Aladdin, Fingal, had one" .)