Genuine new Lloyd Webber musical mounted (6)
I believe the answer is:
native
'genuine' is the definition.
(thesaurus)
'new lloyd webber musical mounted' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'lloyd webber musical' becomes 'evita' (musical about Eva Peron).
'mounted' says the letters should be written in reverse (in a down clue, letters mounted up).
'evita' backwards is 'ative'.
'n'+'ative'='NATIVE'
(Other definitions for native that I've seen before include "Indigenous person" , "Endemic - aborigine" , "One who belongs to place by birth" , "Belonging to a particular place by birth" , "In a natural state" .)