Book of words (10)
'book of words' is the definition.
(lexicon can mean a dictionary)
This is the entire clue.
(Other definitions for lexicon that I've seen before include "Dictionary, especially of ancient languages" , "reference work?" , "it has a lot of words" , "Dictionary as of Greek or Hebrew" , "Word reference book" .)
(Other definitions for dictionary that I've seen before include "Ordered book" , "Lexicon" , "Where to find the meaning of life, for example!" , "glossary" , "reference work" .)