Tense, on edge? Not us! (4)
I believe the answer is:
them
'not us' is the definition.
(I've seen this in another clue)
'tense on edge?' is the wordplay.
'tense' becomes 't' (grammatical abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'edge?' becomes 'hem' (a hem is an edge on a piece of cloth).
't'+'hem'='THEM'
(Other definitions for them that I've seen before include "Let -- eat cake" , "Certainly not us, the other lot" , "people objectively" , "(To/from) those people" , "... in another case, they" .)