Breaking resistance getting into trendy group (10)
I believe the answer is:
infraction
'breaking' is the definition.
(thesaurus)
'resistance getting into trendy group' is the wordplay.
'resistance' becomes 'r' (abbreviation).
'getting into' indicates putting letters inside.
'trendy' becomes 'in'.
'group' becomes 'faction' (faction is a kind of group**).
'in'+'faction'='infaction'
'r' going within 'infaction' is 'INFRACTION'.
(Other definitions for infraction that I've seen before include "Breach of agreement" , "transgression" , "Violation of law" .)