It's polite to include nothing in rudely brief style (7)

(Source: London Evening Standard)

"nothing" ⇒ zero ⇒ the letter "o"

"rudely" ⇒ also "curt"

"brief style" ⇒ some letters of the word "style" ⇒ e.g. "ly"

"o" inside "curt" plus "ly"

Meaning polite

"courtly"