Clue: Nationalist working with editor is totally perplexed in America (9)
Answer: NONPLUSED
Explanation: Definition: perplexed in America

**N** (nationalist) + **ON **("working") + **PLUS **("with") + **Ed.** (editor)
 
*The "in America" in the clue tells us to use the American spelling rather than NONPLUSSED (the British spelling)