Epic radio broadcast is irregular (9)
I believe the answer is:
aperiodic
'is irregular' is the definition.
(not occurring regularly or periodically)
'epic radio broadcast' is the wordplay.
'broadcast' indicates an anagram (letters scattered or cast around).
'epic'+'radio'='epicradio'
'epicradio' anagrammed gives 'APERIODIC'.
(Other definitions for aperiodic that I've seen before include "at irregular intervals" , "without regular metre?" .)