Brass behind in test broadcast (8)
I believe the answer is:
trumpets
'brass' is the definition.
(I've seen this in another clue)
'behind in test broadcast' is the wordplay.
'behind' becomes 'rump' (informal term for the buttocks).
'in' indicates putting letters inside.
'broadcast' indicates anagramming the letters (letters scattered or cast around).
'test' anagrammed gives 'tets'.
'rump' going within 'tets' is 'TRUMPETS'.
(Other definitions for trumpets that I've seen before include "Brass musical instruments" , "Proclaims loudly like elephant sounds" , "Brass wind instruments" .)