Cavernous opening (3)
I believe the answer is:
maw
'cavernous opening' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for maw that I've seen before include "Stomach of men and animals" , "Mouth of a vicious animal" , "Mouth (animal's)" , "Jaws or mouth of a voracious animal" , "Gaping mouth of voracious animal" .)