Check blunder in endless easing of international tension (9)
I believe the answer is:
deterrent
'check' is the definition.
(thesaurus)
'blunder in endless easing of international tension' is the wordplay.
'blunder' becomes 'err' (to err is to make a mistake or blunder).
'in' is an insertion indicator.
'endless' means to remove the last letter.
'easing of international tension' becomes 'detente' (I've seen this before).
'detente' with its last letter removed is 'detent'.
'err' placed inside 'detent' is 'DETERRENT'.
(Other definitions for deterrent that I've seen before include "barrier" , "Eg, a nuclear weapon" , "Something discouraging action" , "Something designed to be off-putting" , "Something intended to discourage" .)