Brash couple, initially livening up (4)
I believe the answer is:
loud
'brash' is the definition.
(I've seen this before)
'couple initially livening up' is the wordplay.
'couple' becomes 'duo' ('duo' can be a synonym of 'couple'**).
'initially' says to take the initial letters.
'up' is a reversal indicator (in down clue: letters go upwards).
The initial letter of 'livening' is 'l'.
'duo'+'l'='duol'
'duol' written backwards gives 'LOUD'.
(Other definitions for loud that I've seen before include "Tastelessly showy" , "Audibly, or visually, obtrusive" , "Tasteless; of great volume" , "This speaker is" , "strongly audible" .)