Soiled apparel remains in flight (7)
I believe the answer is:
washing
'soiled apparel' is the definition.
(dirty clothing waiting to be washed)
'remains in flight' is the wordplay.
'remains' becomes 'ash' (I've seen this before).
'in' means one lot of letters goes inside another.
'flight' becomes 'wing' (wing can mean flight, as in 'on the wing').
'ash' placed into 'wing' is 'WASHING'.
(Other definitions for washing that I've seen before include "(and 18) Domestic appliance" , "Cleaning - laundry" , "Cleansing" , "Cleaning with soap and water" .)