Insurrection is occurring in the arena (6)
I believe the answer is:
rising
'insurrection' is the definition.
(I know that insurrection can be written as rising)
'occurring in the arena' is the wordplay.
'occurring' becomes 'si' (I can't explain this - if you can you should give a lot more credence to this answer).
'in the' is an insertion indicator.
'arena' becomes 'ring' (I've seen this in another clue).
'si' inserted inside 'ring' is 'RISING'.
'is' is the link.
Can you help me to learn more?
(Other definitions for rising that I've seen before include "Upwardly mobile" , "Upward rebellion?" , "Ascension" , "Revolt; coming up" , "working one's way up" .)