I remember the story, which is as follows:
A humble tailor (with his disabled wife as help) on the outskirts of London was suddenly visited by a couple of men in suits who closed his shop and told him he has a special assignment. They introduced him to a centaur type alien, who speaks English just fine with a translator, and they need him to whip up something so the alien can be presented to Her Majesty at a formal function.
The tailor refused to make a "glorified horse blanket" and ended up spending two weeks making jackets, pantaloons, shirts, even undies, after having a fit with the MI5 guy. But the end result was remarkable, even though the wife thought the tailor was just joking about the alien (you're making it for a stage play, right?)
I am reasonably sure it was one of Jerry Pournelle's collections "There Will Be War" or "Star Empire" but I can't remember which one.
A little help?