Flat passage in requiem, initially erased (9)
I believe the answer is:
apartment
'flat' is the definition.
(I know that flat can be written as apartment)
'passage in requiem initially erased' is the wordplay.
'passage' becomes 'part' (part can mean a passage or section).
'in' indicates putting letters inside.
'requiem' becomes 'lament' (synonyms).
'initially erased' means to remove the first letter.
'lament' with its initial letter taken away is 'ament'.
'part' inserted inside 'ament' is 'APARTMENT'.
(Other definitions for apartment that I've seen before include "Room ? flat" , "Accommodation" , "property" , "Suite of rooms to live in" , "19 across" .)