Volcano's mouth (6)
I believe the answer is:
'volcano's mouth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for crater that I've seen before include "Great hole as on top of volcano" , "Caldera" , "Bowl-shaped mouth of volcano" , "Lunar surface dent" , "Fissure in the earth's crust" .)