Volcano mouth (6)
'volcano mouth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for crater that I've seen before include "Fissure in the earth's crust" , "Feature visible on the moon" , "Pit resulting from impact or explosion" , "Great hole as on top of volcano" , "Volcano's mouth" .)
(Other definitions for caldera that I've seen before include "Basin-shaped crater at top of volcano" , "Result of eruption" , "Large volcanic crater -- lead car (anag)" , "Big hole in rock" , "Large volcanic crater - race lad (anag)" .)