Cavern (6)
I believe the answer is:
grotto
'cavern' is the definition.
(I know that grotto is a type of cavern)
This is the entire clue.
(Other definitions for grotto that I've seen before include "Fancy cave" , "In company (with); over length of" , "Cavern" , "Small cave, perhaps with religious connotations" , "Ornamental cave" .)