Diplomacy after Radio Three broadcast is given severe warning (4,3,4,3)
I believe the answer is:
read the riot act
'severe warning' is the definition.
(I've seen this before)
'diplomacy after radio three broadcast is given' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'after' says to put letters next to each other.
'broadcast' indicates anagramming the letters (letters scattered or cast around).
'is given' says to put letters next to each other.
'radio' next to 'three' is 'radiothree'.
'radiothree' is an anagram of 'readtherio'.
'tact' after 'readtherio' is 'READ THE RIOT ACT'.
(Other definitions for read the riot act that I've seen before include "severe warning" , "Wag one's finger" .)