Refer to plot on the radio (4)
I believe the answer is:
cite
'refer to' is the definition.
(synonyms)
'plot on the radio' is the wordplay.
'plot' becomes 'site' (as in a plot of land).
'on the radio' indicates a 'sounds like' (homophone) clue.
'site' is a homophone of 'CITE'.
(Other definitions for cite that I've seen before include "Quote as authority" , "Quote or refer to" , "Quote (a reference)" , "Give as an example" , "Mention as example or quote" .)