A pound's a reduced sum for a book (5)
I believe the answer is:
album
'book' is the definition.
(album is a kind of book)
'a pound's a reduced sum' is the wordplay.
'pound' becomes 'lb' (abbreviation for the unit of weight).
'a reduced' means to remove the first letter.
'sum' with its initial letter taken away is 'um'.
'a'+'lb'+'um'='ALBUM'
'for a' is the link.
(Other definitions for album that I've seen before include "Book of collections" , "Cuttings book" , "It contains multiple musical tracks" , "Book of blank pages for collectables, memorabilia etc." , "Old LP - book of photos" .)