Check blunder in unfinished easing of international tension (9)
I believe the answer is:
deterrent
'check' is the definition.
(thesaurus)
'blunder in unfinished easing of international tension' is the wordplay.
'blunder' becomes 'err' (I've seen this before).
'in' is an insertion indicator.
'unfinished' means to remove the last letter.
'easing of international tension' becomes 'detente' (I've seen this before).
'detente' with its last letter taken off is 'detent'.
'err' put within 'detent' is 'DETERRENT'.
(Other definitions for deterrent that I've seen before include "It acts to discourage" , "Something designed to be off-putting" , "Something that puts people off" , "Weapon to discourage attack" , "It restrains" .)