Crater roughly enclosing tree (7)
I believe the answer is:
caldera
'crater' is the definition.
(I've seen this before)
'roughly enclosing tree' is the wordplay.
'roughly' becomes 'ca'.
'enclosing' 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 "Result of eruption" , "Large volcanic crater - race lad (anag)" , "Basin-shaped crater at top of volcano" , "Large volcanic crater -- lead car (anag)" , "Big hole in rock" .)