Joke about northern band (4)
I believe the answer is:
gang
'band' is the definition.
(I've seen this before)
'joke about northern' is the wordplay.
'joke' becomes 'gag' (a gag is a joke or witticism).
'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 "Maybe criminal group" , "Organised group of labourers" , "Association of criminals" , "Posse" , "Go (Scot)" .)