Principal opponent many cheer wildly (9)
I believe the answer is:
archenemy
'principal opponent' is the definition.
'archenemy' can be an answer for 'opponent' (I have seen 'Chief opponent ' mean 'archenemy' so perhaps 'opponent' could also mean 'archenemy'). I am not certain of the 'principal' bit.
'many cheer wildly' is the wordplay.
'wildly' is an anagram indicator.
'many'+'cheer'='manycheer'
'manycheer' is an anagram of 'ARCHENEMY'.
Can you help me to learn more?
(Other definitions for archenemy that I've seen before include "Main adversary" , "Cheery man can be greatest foe" , "Chief foe" , "most dangerous opponent" , "As Holmes regarded Moriarty" .)