What does the cryptic crossword clue "Ruth uses a different reference book (9)" mean?

The definition is "reference book". The wordplay uses an anagram. anagram of RUTH + USES + A = THESAURUS; definition: "reference book" The answer is THESAURUS.

Ruth uses a different reference book (9)

Daily Mail Cryptic #16266 | 14 Down | 2020-03-06 flower

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Daily Mail Cryptic #16435 (2020-10-29) Guardian Cryptic #23996 (2007-02-09)