A riot out of proportion (5)
I believe the answer is:
ratio
'proportion' is the definition.
(I know that proportion can be written as ratio)
'a riot out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'a'+'riot'='ariot'
'ariot' anagrammed gives 'RATIO'.
'of' is the 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" .)