Writer in the end decided to fill a sheet for attachment (9)
I believe the answer is:
appendage
'for attachment' is the definition.
(thesaurus)
'writer in the end decided to fill a sheet' is the wordplay.
'writer' becomes 'pen' (something which writes).
'in the' indicates putting letters inside.
'end' suggests the final letters.
'to fill' is an insertion indicator.
'sheet' becomes 'page' (sheet of paper).
The final letter of 'decided' is 'd'.
'a'+'page'='apage'
'd' placed within 'apage' is 'apdage'.
'pen' placed into 'apdage' is 'APPENDAGE'.
(Other definitions for appendage that I've seen before include "An outgrowth" , "Attached item" , "Thing attached to another" , "Something attached to a larger thing" , "It's added or attached to a larger or more important part" .)