Volcanic crater about to swallow tree (7)
I believe the answer is:
caldera
'volcanic crater' is the definition.
(I've seen this in another clue)
'about to swallow tree' is the wordplay.
'about' becomes 'ca' (short for circa).
'to swallow' means one lot of letters goes inside another.
'tree' becomes 'alder' (alder is a kind of tree).
'ca' placed around 'alder' is 'CALDERA'.
(Other definitions for caldera that I've seen before include "Result of eruption" , "Basin-shaped crater at top of volcano" , "Large volcanic crater - race lad (anag)" , "Big hole in rock" , "Large volcanic crater -- lead car (anag)" .)