Music genre heard racing across the Atlantic (4)
I believe the answer is:
indy
'racing across the atlantic' is the definition.
(short for the Indianapolis 500 car race)
'music genre heard' is the wordplay.
'music genre' becomes 'indie'.
'heard' indicates a 'sounds like' (homophone) clue.
'indie' sounds like 'INDY'.
(Other definitions for indy that I've seen before include "- - Car racing (in US)" , "Form of competition between drivers" , "US car race" , "Type of motor racing" , "Abandon, discard" .)