Right to stop plebs forming ruck (5)
I believe the answer is:
scrum
'ruck' is the definition.
(I've seen this before)
'right to stop plebs' is the wordplay.
I cannot quite understand how this works, but
'right' could be 'r' (common abbreviation) and 'r' is found in the answer.
The remaining letters 'scum' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'forming' acts as a link.
Can you help me to learn more?
(Other definitions for scrum that I've seen before include "riotous struggle" , "Disorderly mob" , "Rugby set-piece" , "Jostling crowd" , "Fracas" .)