What 21 did in radio report is sad (4)
I believe the answer is:
blue
'sad' is the definition.
('blue' can be similar in meaning to 'sad')
I don't understand the remainder of the clue.
(Other definitions for blue that I've seen before include "sometimes heard of with little boy" , "Sky-coloured" , "Dispirited in cerulean way" , "The colour of melancholy" , "Miserable - cerulean" .)