... not one in three broadcast (7)
I believe the answer is:
neither
'not one in three broadcast' is the definition.
Both the definition and answer are adverbs. Maybe there's an association between them I don't understand?
'in three broadcast' is the wordplay.
'broadcast' indicates anagramming the letters (letters scattered or cast around).
'in'+'three'='inthree'
'inthree' is an anagram of 'NEITHER'.
Can you help me to learn more?
(Other definitions for neither that I've seen before include "double negative?" , "Not one of two" , "The rein (anag.)" , "Not one thing or the other" , "None of the two" .)