One Mr Opera is replaced as concert master (10)
I believe the answer is:
impresario
'concert master' is the definition.
(someone funding or organising concerts)
'one mr opera is replaced' is the wordplay.
'one' becomes 'i' (Roman numeral).
'replaced' indicates an anagram.
'mr'+'opera'+'is'='mroperais'
'mroperais' anagrammed gives 'mpresario'.
'i'+'mpresario'='IMPRESARIO'
'as' is the link.
(Other definitions for impresario that I've seen before include "theatre director" , "Showman" , "Organiser of musical events etc" , "Perhaps the Met's manager" , "Theatrical organiser" .)