Criticism involving northern side (5)
I believe the answer is:
flank
'side' is the definition.
(thesaurus)
'criticism involving northern' is the wordplay.
'criticism' becomes 'flak' (flak is a kind of criticism).
'involving' is an insertion indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'flak' placed around 'n' is 'FLANK'.
(Other definitions for flank that I've seen before include "Haunch" , "Side of an animal" .)