Wait, didn't Jerry use to teach English or something along those lines? I feel like the title should read "Tons of INFI is Flying Out of the Doors!" Must be deliberate... let's see...
Is/are is a subject/verb agreement error based on quantity... if INFI is the same in singular and plural form, that is! So we can assume this might refer to something else where there may be confusion about whether the singular and plural are the same word, but in fact, the word follows other vague pluralization rules--that is, when it's made plural, we must change the word, but not by simply adding an "s" to it.
Meanwhile, the omitted "of" is a preposition that would indicate movement. So in this instance, we are emphasizing the rigidity or the unpredictability of motion. Flying is a very unpredictable motion for most things! Also, the placement of this word can be moved; i.e., even though this clue came second, the word can/should be moved to first in the answer...
"is"... "of"... If we combine the words into "isof" rearrange the letters, we find it's an anagram for "fois," which sounds very similar to the French "foie," which means liver. This is most commonly used when connected to the dish, "foie gras." Foie gras is most often the liver of a goose!
Ladies and gentlemen, THE BUSSE COMBAT FLYING GOOSE IS COMING!!!