Northern editor keeps rats -- say it doesn't matter (5,4)
I believe the answer is:
never mind
'it doesn't matter' is the definition.
(I've seen this before)
'northern editor keeps rats' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'editor' becomes 'ed' (abbreviation).
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'rats' becomes 'vermin' (I've seen this before).
'ed' going around 'vermin' is 'evermind'.
'n'+'evermind'='NEVER MIND'
'say' is the link.
(Other definitions for never mind that I've seen before include "Forget" , "It doesn't matter" , "Don't worry" , "who cares?" .)