Indistinguishable dialect in broadcast (9)
I believe the answer is:
identical
'indistinguishable' is the definition.
(synonyms)
'dialect in broadcast' is the wordplay.
'broadcast' indicates anagramming the letters.
'dialect'+'in'='dialectin'
'dialectin' anagrammed gives 'IDENTICAL'.
(Other definitions for identical that I've seen before include "lacking distinction" , "Matching" , "Duplicate" , "Aid client to be the very same" , "not distinguished" .)