Customer in Celt's broadcast (6)
I believe the answer is:
client
'customer' is the definition.
(synonyms)
'in celt's broadcast' is the wordplay.
'broadcast' is an anagram indicator (letters scattered or cast around).
'in'+'celt'='incelt'
'incelt' is an anagram of 'CLIENT'.
(Other definitions for client that I've seen before include "One represented by lawyer" , "Eg, lawyer's customer" , "Patron" , "Professional person's customer" , "Customer of professional person" .)