Conciseness (9)
'conciseness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for brevity that I've seen before include "Succinctness" , "considered the essence of wit" , "Terseness" , "Conciseness" , "The soul of wit?" .)
(Another definition for briefness that I've seen is " Concision".)