Gibraltar sent new threat of military action (5,8)
I believe the answer is:
sabre rattling
'threat of military action' is the definition.
'gibraltar sent new' is the wordplay.
'new' indicates an anagram.
'gibraltar'+'sent'='gibraltarsent'
'gibraltarsent' is an anagram of 'SABRE-RATTLING'.
(Other definitions for sabre rattling that I've seen before include "Aggressive display of military force" , "by jingo!" , "Threatening force" , "aggressive behaviour?" , "Aggressive display of military power" .)