Crashed down? All in flat, depressed region ... (7)
I believe the answer is:
lowland
'flat depressed region' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'crashed down? all' is the wordplay.
'crashed' indicates anagramming the letters.
'down'+'all'='downall'
'downall' with letters rearranged gives 'LOWLAND'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for lowland that I've seen before include "Region not as high as other areas" , "Terrain of little altitude" , "No high countryside" , "Area with no high points" , "The Fens, perhaps" .)