Joke about northern band of delinquents (4)
I believe the answer is:
gang
'band of delinquents' is the definition.
(I've seen this before)
'joke about northern' is the wordplay.
'joke' becomes 'gag' (synonyms).
'about' is an insertion indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'gag' enclosing 'n' is 'GANG'.
(Other definitions for gang that I've seen before include "Crew, band" , "Band of delinquents" , "A lot of labourers" , "Association of criminals" , "Criminal association" .)