Long for a letterhead to service book (6)
I believe the answer is:
missal
'service book' is the definition.
(thesaurus)
'long for a letterhead' is the wordplay.
'long for' becomes 'miss' (I've seen this in another clue).
'letterhead' becomes 'l' (head letter of 'letter').
'miss'+'a'+'l'='MISSAL'
'to' acts as a link.
(Other definitions for missal that I've seen before include "Mass medium" , "guide for the services" , "Catholic mass text book" , "Book of church texts" , "Book of prayers and rites" .)