Area with dry rot rising (6)
I believe the answer is:
sector
'area' is the definition.
(I know that area can be written as sector)
'with dry rot rising' is the wordplay.
'with' means one lot of letters go next to another.
'dry' becomes 'sec' (similar in meaning).
'rising' says the letters should be written backwards (in a down clue, letters go up).
'rot' backwards is 'tor'.
'sec' next to 'tor' is 'SECTOR'.
(Other definitions for sector that I've seen before include "region" , "Part of circle between radii" , "Division or part" , "Military sub-area" , "Area covered by corset" .)