Tense at the border (2,4)
I believe the answer is:
on edge
'tense' is the definition.
(thesaurus)
'at the border' is the wordplay.
'at' becomes 'on' (location).
'the border' becomes 'edge' ('edge' can be a synonym of 'border').
'on'+'edge'='ON EDGE'
(Other definitions for on edge that I've seen before include "Tense and irritable, with nerves stretched" , "Nervously excited" , "All agog" , "Anxious, nervy" , "Twitchy" .)