Disaffection of people in a Regent St riot (12)
I believe the answer is:
estrangement
'disaffection' is the definition.
('estrangement' can be a synonym of 'disaffection')
'people in a regent st riot' is the wordplay.
'people' becomes 'men' (man is a kind of person).
'in' indicates putting letters inside.
'riot' indicates an anagram.
'a'+'regent'+'st'='aregentst'
'aregentst' anagrammed gives 'estranget'.
'men' inserted within 'estranget' is 'ESTRANGEMENT'.
'of' acts as a link.
(Other definitions for estrangement that I've seen before include "division" , "lack of good relationship" , "Alienation" , "Result of splitting up" .)