Of short duration (5)
'of short duration' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for brief that I've seen before include "Short or concise" , "Terse" , "Document stating the facts of a legal case" , "Succinct" , "Ephemeral" .)
(Other definitions for transitory that I've seen before include "Passing" , "Ephemeral" , "Here-today-gone-tomorrow" , "Short-lived" , "Fleeting" .)
(Other definitions for transient that I've seen before include "gone in a trice!" , "Ephemeral" , "Fleeting" , "fugitive" , "Lasting little time" .)