Volcanic opening about to swallow tree (7)
I believe the answer is:
caldera
'volcanic opening' is the definition.
'about to swallow tree' is the wordplay.
'about to' becomes 'ca' (I've seen this in other clues).
'swallow' is an insertion indicator.
'tree' becomes 'alder' (alder tree is a kind of tree**).
'ca' going around 'alder' is 'CALDERA'.
(Other definitions for caldera that I've seen before include "Big hole in rock" , "Large volcanic crater -- lead car (anag)" , "Basin-shaped crater at top of volcano" , "Large volcanic crater - race lad (anag)" , "Result of eruption" .)