Tense at the border (2,4)
I believe the answer is:
Here is my best explanation:
'tense' is the definition.
(I've seen this before)
'at the border' is the subsidiary indication.
'at' becomes 'on' (location).
'the border' becomes 'edge' ('edge' can be a synonym of 'border').
(Other definitions for on edge that I've seen before include "Very nervous or tense", "stressed", "Highly strung", "Anxious, nervy", "Under stress", "not yet having been invited into the circle?", "Cliff-top walker may be", "Fidgety, nervous", "'Tense, nervous (2,4)'" and "looking down at Much Wenlock?".)