Cleaning part of building, coming across burnt remains (7)
I believe the answer is:
washing
'cleaning' is the definition.
(wash can mean to clean)
'part of building coming across burnt remains' is the wordplay.
'part of building' becomes 'wing' ('wing' is part of 'building').
'coming across' means one lot of letters goes inside another.
'burnt remains' becomes 'ash' (remains of a fire).
'wing' placed around 'ash' is 'WASHING'.
(Other definitions for washing that I've seen before include "Cleaning with soap and water" , "Cleaning - laundry" , "Cleansing" , "(and 18) Domestic appliance" .)