Popular part in law-breaking (10)
I believe the answer is:
infraction
'law-breaking' is the definition.
(infraction is a kind of law-breaking)
'popular part' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'part' becomes 'fraction' (fraction is a kind of part).
'in'+'fraction'='INFRACTION'
'in' is the link.
(Other definitions for infraction that I've seen before include "transgression" , "Violation of law" , "Breach of agreement" .)