Genuine monarch’s principal possession? (5)
I believe the answer is:
realm
'possession?' is the definition.
(thesaurus)
'genuine monarch's principal' is the wordplay.
'genuine' becomes 'real' (similar in meaning).
'principal' indicates taking the first letters (the principal letter is the first letter).
The first letter of 'monarchs' is 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "World" , "Area of knowledge under discussion" , "Kingdom, or field of activity" , "field of study" , "region" .)