Broadcast relating to number one cause of boredom (5)
I believe the answer is:
ennui
'boredom' is the definition.
(synonyms)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for ennui that I've seen before include "Feeling when tired of life?" , "French boredom and lassitude" , "What's tedious and instils such listlessness" , "having nothing to do" , "Bored listlessness" .)