(Not a teacher) Before the experts answer you, may I make a few suggestions? Yes, in high-school level English analysis, most teachers would probably just call "to" a preposition or simply a "sign" of the infinitive, They would tell their students it's not important what you call it. On a higher level of study, you might be told the following: (l) Sometimes "to" is a real preposition that does NOT permit only a main verb (We look forward TO your visit; We look forward TO meetING you. In your example, you cannot say "Something to food" or "Something to eatING." Therefore, we can conclude that "to" in "Something to eat" is, indeed, a infinitival particle. (I wish to credit The Oxford Dictionary of English Grammar for this information.) Let's see what the experts say when they answer you.