Spaniard's agreement to enter boxing arena causes revolt (6)
I believe the answer is:
rising
'revolt' is the definition.
(I know that revolt can be written as rising)
'spaniard's agreement to enter boxing arena' is the wordplay.
'spaniard's agreement' becomes 'si' (I can't justify this - if you can you should give a lot more credence to this answer).
'to enter' is an insertion indicator.
'boxing arena' becomes 'ring' (boxing ring).
'si' inserted into 'ring' is 'RISING'.
'causes' is the link.
Can you help me to learn more?
(Other definitions for rising that I've seen before include "Revolt; increasing" , "Revolt; coming up" , "Moving upwards" , "Armed revolt against authority" , "A revolt; getting up" .)