An anti-fairy one, of course (the clovers being against their glamors), but then a character (not the one making it) was pondering how it helped the victims of fairy magic by positing nutritional deficiencies.
Ah the wonders of magica, the use of the occult properties of things. ("Occult" meaning, after all, "hidden.") It's in full force in the modern world, we just call it "science."
Then -- four-leafed clovers! He'd founder on explaining why four-leafed, not three-leafed, clovers are needed.
Unless he gets into subtle changes in the proteins stemming from the mutation.