Cleaning wood in part of a mansion (7)
I believe the answer is:
washing
'cleaning' is the definition.
(wash can mean to clean)
'wood in part of a mansion' is the wordplay.
'wood' becomes 'ash' (wood from an ash tree).
'in' indicates putting letters inside.
'part of a mansion' becomes 'wing' (I've seen this in another clue).
'ash' put within 'wing' is 'WASHING'.
(Other definitions for washing that I've seen before include "Cleaning with soap and water" , "Cleansing" , "Cleaning - laundry" , "(and 18) Domestic appliance" .)