King among plebs causing ruck (5)
I believe the answer is:
scrum
'ruck' is the definition.
(I've seen this before)
'king among plebs' is the wordplay.
'king' becomes 'CR' (short for Carolus Rex, King Charles in Latin).
'among' indicates putting letters inside.
'plebs' becomes 'sum' (I can't explain this - if you can you should give a lot more credence to this answer).
'cr' put into 'sum' is 'SCRUM'.
'causing' is the link.
Can you help me to learn more?
(Other definitions for scrum that I've seen before include "Rugby set-piece" , "Fracas" , "Rugby huddle" , "Melee" , "Rugby get-together" .)