Confirm trouble in northern county (4,4)
I believe the answer is:
nail down
'confirm' is the definition.
(to nail down is to confirm)
'trouble in northern county' is the wordplay.
'trouble' becomes 'ail' (synonyms).
'in' means one lot of letters goes inside another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'county' becomes 'down' (County Down in Northern Ireland).
'n'+'down'='ndown'
'ail' going within 'ndown' is 'NAIL DOWN'.
(Other definitions for nail down that I've seen before include "Bind to a promise, using hammer and metal pin" , "Firmly fix" , "Identify (something) precisely" , "make final" , "An old win (anag.)" .)