Excellent policeman joins head of broadcasting (6)
I believe the answer is:
superb
'excellent' is the definition.
(I've seen this before)
'policeman joins head of broadcasting' is the wordplay.
'policeman' becomes 'super' (short for superintendant).
'joins' is a charade indicator (letters next to each other).
'head of' says to take the initial letters.
The first letter of 'broadcasting' is 'b'.
'super'+'b'='SUPERB'
(Other definitions for superb that I've seen before include "Splendid, brilliant" , "Of surpassing excellence" , "Magnificent, quite splendid" , "choice" , "fine" .)