Notice function on the radio (4)
I believe the answer is:
sign
'notice' is the definition.
(I've seen this in another clue)
'function on the radio' is the wordplay.
'function' becomes 'sine' (mathematical function).
'on the radio' indicates a 'sounds like' (homophone) clue.
'sine' sounds like 'SIGN'.
(Other definitions for sign that I've seen before include "Endorse" , "Show" , "Engage by written agreement" , "add name" , "Leo, for instance" .)