Girl with a large service book (6)
I believe the answer is:
missal
'service book' is the definition.
(thesaurus)
'girl with a large' is the wordplay.
I cannot quite see how this works, but
'girl' could be 'miss' (term of address for an unmarried woman) and 'miss' is found within the answer.
'a' is within the answer.
'large' could be 'l' (eg in clothes sizes) and 'l' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for missal that I've seen before include "A book of prayers used in mass" , "Church book covering all services in the year" , "Book of church texts" , "Catholic service book" , "A book containing prayers of the Mass" .)