Much arena activity for us (5,4)
I believe the answer is:
human race
'us' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'much arena activity' is the wordplay.
'activity' indicates an anagram (letters become active).
'much'+'arena'='mucharena'
'mucharena' with letters rearranged gives 'HUMAN RACE'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for human race that I've seen before include "See 8" , "Mankind" , "All of us" , "Homo sapiens" , "We are all involved" .)