Large and small guitar book (4)
I believe the answer is:
luke
'book' is the definition.
(I know that Luke is a book)
'large and small guitar' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'and' means one lot of letters go next to another.
'small guitar' becomes 'uke' (I've seen this in another clue).
'l'+'uke'='LUKE'
(Other definitions for luke that I've seen before include "One of the Apostles" , "Matthew, Mark, - - and John" , "Traditionally the author of the third Gospel" , "Third NT book" , "Physician gospeller" .)