Nervous — tense (6)
This clue is a double definition.
'nervous' is the first definition.
(on edge can mean nervous or tense)
'tense' is the second definition.
(on edge can mean tense or stressed)
(Other definitions for on edge that I've seen before include "Tense and irritable, with nerves stretched" , "Apprehensive" , "close to border" , "stressed" , "Twitchy" .)