Audibly mark offbeat music (3)
I believe the answer is:
ska
'offbeat music' is the definition.
'audibly mark' is the wordplay.
'audibly' shows a homophone (sound like).
'mark' becomes 'scar' ('scar' can be a synonym of 'mark').
'scar' sounds like 'SKA'.
(Other definitions for ska that I've seen before include "Dancehall music" , "Jamaican music, forerunner of reggae" , "Noted genre" , "Jamaican music style" , "popular music" .)