Complete reversal of Northern Ireland diplomacy (6)
I believe the answer is:
intact
'complete' is the definition.
(intact can mean complete or undamaged)
'reversal of northern ireland diplomacy' is the wordplay.
'reversal of' shows that the letters should be reversed in order.
'northern ireland' becomes 'NI' (abbreviation).
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'ni' written backwards gives 'in'.
'in'+'tact'='INTACT'
(Other definitions for intact that I've seen before include "Not damaged in any way" , "Unbroken and unimpaired" , "Entire, untouched" , "Inviolate" , "Whole and undamaged" .)