Forerunner of rocksteady (3)
I believe the answer is:
ska
'forerunner of rocksteady' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ska that I've seen before include "popular music" , "Dancehall music" , "Jamaican music style" , "Music genre" , "Jamaican music, forerunner of reggae" .)