Volcano's mouth (6)
I believe the answer is:
crater
'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" .)