Zealous and sincere -- how he altered in a riot (12)
I believe the answer is:
wholehearted
'zealous and sincere' is the definition.
(wholehearted means sincerely committed)
'how he altered in a riot' is the wordplay.
'in a riot' is an anagram indicator.
'how'+'he'+'altered'='howhealtered'
'howhealtered' anagrammed gives 'WHOLEHEARTED'.
(Other definitions for wholehearted that I've seen before include "Unqualified" , "Completely sincere and committed" , "Unreserved - affectionate" , "With enthusiastic devotion" , "Earnest" .)