A condiment knocked over reference book (5)
I believe the answer is:
atlas
'reference book' is the definition.
(atlas is a kind of reference book)
'a condiment knocked over' is the wordplay.
'condiment' becomes 'salt' (I've seen this before).
'knocked over' shows that the letters should be reversed in order.
'salt' backwards is 'tlas'.
'a'+'tlas'='ATLAS'
(Other definitions for atlas that I've seen before include "Large-format book" , "Book of geography" , "Album of maps" , "African mountain range" , "travel book" .)