Northern Ireland flipping certain to get guarantee (6)
I believe the answer is:
insure
'guarantee' is the definition.
(I know that guarantee can be written as insure)
'northern ireland flipping certain' is the wordplay.
'northern ireland' becomes 'NI' (abbreviation).
'flipping' says the letters should be written backwards.
'certain' becomes 'sure' (synonyms).
'ni' backwards is 'in'.
'in'+'sure'='INSURE'
'to get' acts as a link.
(Other definitions for insure that I've seen before include "Guarantee against loss" , "Protect value through contract" , "Make certain" , "Provide protection against risk or damage" , "Guarantee; make secure" .)