Broadcast beginning with piano duet (4)
I believe the answer is:
pair
'duet' is the definition.
(synonyms)
'broadcast beginning with piano' is the wordplay.
'broadcast' becomes 'air' (to air on TV).
'beginning with' says to put letters next to each other (some letters go at the beginning of others).
'piano' becomes 'p' (musical abbreviation).
'air' after 'p' is 'PAIR'.
(Other definitions for pair that I've seen before include "item" , "Twosome" , "Twins" , "Couple - wed" , "Two of a kind, as in cards" .)