This one is a novella, titled "The Reluctant Op", and it's in the Sept/Oct issue of Ellery Queen Mystery Magazine, now available. At least I think it's available; my copies came in the mail yesterday.

http://www.barbarapaul.com/shortmys2.html#reluct

The only name on the cover is "Ellery Queen", along with a picture of Dannay and Lee. Because this is an anniversary issue -- EQ is seventy years old.