Required by etiquette (2,7)
I believe the answer is:
de rigueur
'required by etiquette' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for de rigueur that I've seen before include "necessary" , "Proper" , "Required by strict etiquette or fashion (French)" , "Strictly" , "Required by custom or etiquette" .)