Cleaning wood in the annex (7)
I believe the answer is:
washing
'cleaning' is the definition.
(washing is a kind of cleaning)
'wood in the annex' is the wordplay.
'wood' becomes 'ash' (ash is a kind of wood).
'in' is an insertion indicator.
'the annex' becomes 'wing' (synonyms).
'ash' inserted into 'wing' is 'WASHING'.
(Other definitions for washing that I've seen before include "Cleaning with soap and water" , "(and 18) Domestic appliance" , "Cleansing" , "Cleaning - laundry" .)