Authenticity exists in a kingdom (7)
I believe the answer is:
realism
'authenticity' is the definition.
(thesaurus)
'exists in a kingdom' is the wordplay.
'exists' becomes 'is' (if something 'is', it exists).
'in' is an insertion indicator.
'a kingdom' becomes 'realm' (kingdom is a type of country or realm).
'is' put into 'realm' is 'REALISM'.
(Other definitions for realism that I've seen before include "Practicality of mailers" , "Pragmatic view" , "Representation in an accurate and true to life way" , "Acceptance of life as it is" , "Pragmatic approach to the facts" .)