Book a large seat (5)
I believe the answer is:
album
'book' is the definition.
(album is a kind of book)
'a large seat' is the wordplay.
'large' becomes 'l'.
'seat' becomes 'bum' ('bum' can be a synonym of 'seat').
'a'+'l'+'bum'='ALBUM'
(Other definitions for album that I've seen before include "A music record or a book of photos" , "Book for stamps/photos" , "It contains multiple musical tracks" , "CD or photo book" , "Blank book for keeping stamps, photographs etc" .)