A hollow in the earth (4)
I believe the answer is:
cave
'a hollow in the earth' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cave that I've seen before include "Aladdin, Fingal, had one" , "part of rocky coastline?" , "Natural hollow in side of hill or cliff" , "Hollow in a rock" , "Watch out" .)