Reference books are thus improperly used by one (8)
I believe the answer is:
thesauri
'reference books' is the definition.
(thesaurus is a kind of reference book)
'are thus improperly used by one' is the wordplay.
'improperly' is an anagram indicator (I've seen this in other clues).
'used by' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'are'+'thus'='arethus'
'arethus' is an anagram of 'thesaur'.
'thesaur'+'i'='THESAURI'
(Other definitions for thesauri that I've seen before include "Word books" , "they may aid writers" , "Word founts" , "Dictionaries" , "References" .)