Fail to include a line in service book (6)
I believe the answer is:
missal
'service book' is the definition.
(I know that missal is a type of service book)
'fail to include a line' is the wordplay.
'fail to include' becomes 'miss' (miss out).
'line' becomes 'l' (used when specifying particular lines from a poem).
'miss'+'a'+'l'='MISSAL'
'in' is the link.
(Other definitions for missal that I've seen before include "Religious book" , "Yearbook" , "Service-book" , "Book of prayers and rites" , "book needed to get through service" .)