Oratory of MAs leaving choirmaster confused (8)
I believe the answer is:
rhetoric
'oratory' is the definition.
(rhetoric is a kind of oratory)
'mas leaving choirmaster confused' is the wordplay.
'leaving' indicates named letters should be taken away.
'confused' indicates an anagram.
'choirmaster' with 'mas' taken away is 'choirter'.
'choirter' is an anagram of 'RHETORIC'.
'of' is the link.
(Other definitions for rhetoric that I've seen before include "Grandiloquence, oratory" , "Art of effective speaking" , "Art of persuasive speaking" , "Impressive or exaggerated speech as in public" , "insincere stuff" .)