Book a seat on-line (5)
I believe the answer is:
album
'book' is the definition.
(album is a kind of book)
'a seat on-line' is the wordplay.
'seat' becomes 'bum' (synonyms).
'on' means one lot of letters go next to another.
'line' becomes 'l' (used when specifying particular lines from a poem).
'bum' put after 'l' is 'lbum'.
'a'+'lbum'='ALBUM'
(Other definitions for album that I've seen before include "Long-playing record" , "Book of collections" , "Old LP - book of photos" , "LP, CD; blank book" , "Pictures" .)