America's on edge, I say! (4)
I believe the answer is:
ahem
'i say' is the definition.
(both can be used to get someone's attention)
'america's on edge' is the wordplay.
'america' becomes 'A' (common abbreviation - e.g. in organisation names).
'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).
'a'+'hem'='AHEM'
(Other definitions for ahem that I've seen before include "It's said to gain attention; ah, me (anag.)" , "Throat clearance to attract attention" , "Expression of disapproval" , "I say!" , "Sound made when clearing throat" .)