Crossword Genius

Seems it's instinctive for school head to denounce unorthodox art (6,6)

Ross

I believe the answer is:

second nature

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Irish Times.
Want a hint initially instead of a full solution? Install my app