Treatise followed on the radio (5)
I believe the answer is:
tract
'treatise' is the definition.
(tract is a kind of treatise)
'followed on the radio' is the wordplay.
'followed' becomes 'tracked' (I've seen this before).
'on the radio' indicates a 'sounds like' (homophone) clue.
'tracked' is a homophone of 'TRACT'.
(Other definitions for tract that I've seen before include "Extended area of land" , "Pamphlet, large expanse of land" , "Part of body" , "Pamphlet; area of land" , "Large area; pamphlet" .)