I name Rex mistakenly as analyst (8)
I believe the answer is:
examiner
'analyst' is the definition.
(thesaurus)
'i name rex mistakenly' is the wordplay.
'mistakenly' indicates an anagram.
'i'+'name'+'rex'='inamerex'
'inamerex' is an anagram of 'EXAMINER'.
'as' acts as a link.
(Other definitions for examiner that I've seen before include "Who judges" , "One in charge of test" , "One looks for information" , "Tester or inspector" , "Assessor" .)