Books light metal seats (6)
I believe the answer is:
albums
'books' is the definition.
(album is a kind of book)
'light metal seats' is the wordplay.
'light metal' becomes 'al' (chemical symbol for Aluminium).
'seats' becomes 'bums' ('bum' can be a synonym of 'seat').
'al'+'bums'='ALBUMS'
(Other definitions for albums that I've seen before include "Sets of tracks" , "Books of blank pages, for photographs say" , "Photograph books" , "Blank book (for stamps)" , "Music records or photo books" .)