Tension caused by troubled coward crossing line (4,3)
I believe the answer is:
cold war
'tension' is the definition.
(the cold war was a period of geopolitical tension)
'troubled coward crossing line' is the wordplay.
'troubled' indicates an anagram.
'crossing' is an insertion indicator.
'line' becomes 'l'.
'coward' is an anagram of 'codwar'.
'codwar' placed around 'l' is 'COLD WAR'.
'caused by' acts as a link.
(Other definitions for cold war that I've seen before include "State of tension and hostility" , "Chilly hostility between nations without actual fighting" , "State of tension after 1945" , "Period of East-West political hostility" , "Hostile period after 1945" .)