A riot out of proportion (5)
I believe the answer is:
ratio
'proportion' is the definition.
(I've seen this before)
'a riot out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'a'+'riot'='ariot'
'ariot' is an anagram of 'RATIO'.
'of' acts as a link.
(Other definitions for ratio that I've seen before include "Relationship found in a riot" , "Proportional relation between two numbers" , "Quotient, proportion" , "Proporation, quantitative relation" , "Measure of two relative sizes" .)