Lessee aunt sent out to secure book (9)
I believe the answer is:
subtenant
'lessee' is the definition.
(thesaurus)
'aunt sent out to secure book' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'to secure' indicates putting letters inside (inserted letters are secured inside).
'book' becomes 'b' (abbreviation for book).
'aunt'+'sent'='auntsent'
'auntsent' is an anagram of 'sutenant'.
'sutenant' enclosing 'b' is 'SUBTENANT'.
(Other definitions for subtenant that I've seen before include "renter?" , "he is renting from another leaseholder" .)