Runner jams broadcast (4)
I believe the answer is:
styx
This clue could be a double definition.
'runner' is the first definition.
(mythical river)
'jams broadcast' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Other definitions for styx that I've seen before include "Charon's 26 carried dead souls across this river" , "'River of Hate' in Hades" , "Mythological river in Hades" , "River around Hades" , "The last river one crosses" .)