Small firm with use for northerner (6)
I believe the answer is:
scouse
'northerner' is the definition.
(I've seen this before)
'small firm with use' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'firm' becomes 'co' (a firm is a company).
'with' is a charade indicator (letters next to each other).
's'+'co'+'use'='SCOUSE'
'for' acts as a link.
(Other definitions for scouse that I've seen before include "another city's dialect" , "Liverpool accent" , "What they speak in Liverpool?" , "Liverpudlian accent" , "English dialect spoken in Liverpool" .)