Girl one left alone with a book (6)
I believe the answer is:
missal
'book' is the definition.
(missal is a kind of book)
'girl one left alone with a' is the wordplay.
'girl' becomes 'miss' (I've seen this before**).
'left' indicates named letters should be taken away.
'alone' becomes 'lone' ('lone' can be a synonym of 'alone').
'with' means one lot of letters go next to another.
'lone' with 'one' taken out is 'l'.
'l' put after 'a' is 'al'.
'miss'+'al'='MISSAL'
(Other definitions for missal that I've seen before include "A book containing prayers of the Mass" , "Book of church texts" , "Yearbook" , "Church book covering all services in the year" , "Prayer-book" .)