Function: to spin on the radio (4)
I believe the answer is:
role
'function' is the definition.
('role' can be a synonym of 'function')
'spin on the radio' is the wordplay.
'spin' becomes 'roll'.
'on the radio' shows a homophone (sound like).
'roll' is a homophone of 'ROLE'.
'to' acts as a link.
(Other definitions for role that I've seen before include "Acting part" , "Stage part" , "Duty" , "A part of lore" , "Part played (in life?)" .)