Orchestra's man arranged main orch. (12)
I believe the answer is:
philharmonic
'orchestra's' is the definition.
(thesaurus)
'man arranged main orch' is the wordplay.
'man' becomes 'phil' (**).
'arranged' indicates anagramming the letters.
'main'+'orch'='mainorch'
'mainorch' anagrammed gives 'harmonic'.
'phil'+'harmonic'='PHILHARMONIC'
(Other definitions for philharmonic that I've seen before include "Word found in the title of many orchestras (literally, devoted to music)" , "Music-loving" , "Devoted to music (used in orchestra names)" .)