It's polite to include nothing in rudely brief style (7)
I believe the answer is:
courtly
'it's polite to include' is the definition.
I can't tell whether this definition defines the answer.
'nothing in rudely brief style' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'in' indicates putting letters inside.
'rudely brief' becomes 'curt' (I've seen this before).
'style' becomes 'ly' (I can't explain this - if you can you should believe this answer much more).
'o' placed into 'curt' is 'court'.
'court'+'ly'='COURTLY'
Can you help me to learn more?
(Other definitions for courtly that I've seen before include "Gracious" , "Elegant -- refined" , "Middle Ages code of conduct for couples -- Ely colour TV (anag)" .)