Roughly treated by a gang with a plot (6)
I believe the answer is:
mobbed
'roughly treated' is the definition.
Although both the answer and definition are past participle verbs, I don't see how one could define the other.
'a gang with a plot' is the wordplay.
'a gang' becomes 'mob' ('mob' can be a synonym of 'gang').
'with' is a charade indicator (letters next to each other).
'a plot' becomes 'bed' (a plot of land).
'mob'+'bed'='MOBBED'
'by' is the link.
Can you help me to learn more?
(Other definitions for mobbed that I've seen before include "Crowded round (someone) in disorderly manner" , "Attacked by angry or enthusiastic crowd" , "Set upon by angry crowd" , "Fell on" , "Oppressed by crowd" .)