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