A Norwegian's principal spruce in NI county (6)
I believe the answer is:
antrim
'ni county' is the definition.
(I've seen this before)
'a norwegian's principal spruce' is the wordplay.
'principal' says to take the initial letters.
'spruce' becomes 'trim' (synonyms).
The initial letter of 'norwegians' is 'n'.
'a'+'n'+'trim'='ANTRIM'
'in' acts as a link.
(Other definitions for antrim that I've seen before include "County of Northern Ireland" , "N Ireland county of green glens" , "18 down in Ulster" , "County of northeast Ireland" , "NI town" .)