Part of format the writer used for book (7)
I believe the answer is:
matthew
'for book' is the definition.
(book of the Bible)
'part of format the writer' is the wordplay.
'part of' indicates a hidden word.
'MATTHEW' can be found hidden inside 'format the writer'.
'used' is the link.
(Other definitions for matthew that I've seen before include "This chap" , "Apostle to whom the first Gospel is ascribed" , "Gospel writer" , "Gospel-writer" , "religious text" .)