King stops plebs causing ruck (5)
I believe the answer is:
scrum
'ruck' is the definition.
(I've seen this before)
'king stops plebs' is the wordplay.
'king' becomes 'CR' (short for Carolus Rex, King Charles in Latin).
'stops' indicates putting letters inside (stopping can mean blocking a gap).
'plebs' becomes 'sum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cr' inserted within 'sum' is 'SCRUM'.
'causing' acts as a link.
Can you help me to learn more?
(Other definitions for scrum that I've seen before include "brawl" , "group of forwards" , "Disorderly struggle" , "Scrimmage" , "Rugby pack" .)