Ruin mood and rioting starts (3)
I believe the answer is:
mar
'ruin' is the definition.
(I've seen this before)
'mood and rioting starts' is the wordplay.
'starts' says to take the initial letters.
The first letters of 'mood and rioting' is 'MAR'.
(Other definitions for mar that I've seen before include "when spring growth starts?" , "Damage; month (abbr.)" , "Spoil or deface" , "Spanish main?" , "Spoil something" .)