Just over half of audience madly cheers (5)
I believe the answer is:
adieu
'cheers' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for adieu that I've seen before include "Parting word" , "Comment on leaving" , "12 Down" , "Vale" , "See you later" .)