Top forty in Roman numerals on the radio? (5)
I believe the answer is:
excel
'top' is the definition.
(I've seen this before)
'forty in roman numerals on the radio?' is the wordplay.
'forty in roman numerals' becomes 'XL'.
'on the radio?' shows a homophone (sound like) (I've seen 'on the radio' mean this).
'xl' sounds like 'EXCEL'.
(Other definitions for excel that I've seen before include "Perform extremely well" , "surpass all others" , "Be very good at, better than others" , "Be highly proficient" , "Outdo" .)