Response in an instant on the radio (3)
I believe the answer is:
tic
'response' is the definition.
(I have seen 'Involuntary response ' mean 'tic' so perhaps 'response' could also mean 'tic')
'an instant on the radio' is the wordplay.
'an instant' becomes 'tick' (I've seen this before).
'on the radio' shows a homophone (sound like).
'tick' is a homophone of 'TIC'.
'in' acts as a link.
(Other definitions for tic that I've seen before include "Nervous twitch" , "Localised habitual twitch, especially to the face" , "Convulsive spasm" , "Muscle twitch" , "Facial movement" .)