One who follows somebody on Radio Four, ultimately (9)
I believe the answer is:
successor
'one who follows' is the definition.
(a successor follows someone else)
'somebody on radio four ultimately' is the wordplay.
'somebody' becomes 'success' (a 'somebody' is a successful person).
'on' says to put letters next to each other.
'ultimately' indicates one should take the final letters.
The final letters of 'radio four' are 'or'.
'success'+'or'='SUCCESSOR'
(Other definitions for successor that I've seen before include "one will take your place" , "One who follows another" , "Person who follows next in order" , "for one of the following people" , "One follows next in order" .)