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