Top 40 on the radio (5)
I believe the answer is:
excel
'top' is the definition.
(excel can mean to better or top)
'40 on the radio' is the wordplay.
'40' becomes 'xl' (Roman numerals).
'on the radio' shows a homophone (sound like).
'xl' is a homophone of 'EXCEL'.
(Other definitions for excel that I've seen before include "Outrival" , "Do really well as at exams" , "Perform exceptionally well" , "Be superior to" , "Do particularly well" .)