Mixed chorus here at monastery (12)
I believe the answer is:
charterhouse
'monastery' is the definition.
(I've seen this before)
'mixed chorus here at' is the wordplay.
'mixed' indicates an anagram.
'chorus'+'here'+'at'='chorushereat'
'chorushereat' is an anagram of 'CHARTERHOUSE'.
(Other definitions for charterhouse that I've seen before include "The - of Parma, novel by Stendhal" , "Monastery - public school" .)