Reference fantastic tonic in journal (10)
I believe the answer is:
dictionary
'reference' is the definition.
(I know that dictionary is a type of wordbook)
'fantastic tonic in journal' is the wordplay.
'fantastic' is an anagram indicator (a fantastic or improbable ordering of the letters).
'in' means one lot of letters goes inside another.
'journal' becomes 'diary' (synonyms).
'tonic' anagrammed gives 'ction'.
'ction' placed inside 'diary' is 'DICTIONARY'.
(Other definitions for dictionary that I've seen before include "Where to find the meaning of life, for example!" , "Lexicon" , "glossary" , "reference work" , "Ordered book" .)