Seems it's instinctive for school head to denounce unorthodox art (6,6)
I believe the answer is:
second nature
'art' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I don't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for second nature that I've seen before include "Apparently innate behaviour" , "Almost instinctive behaviour" , "acquired habit?" , "Deeply ingrained tendency" , "it's become instinctive" .)