This'll is a contraction of two parts of speech, this (pronoun) and will (verb).
The main verse is: So bye-bye, miss American pie. Drove my Chevy to the levee, But the levee was dry. And them good old boys were drinkin whiskey and rye Singin, thisll be the day that I die. Thisll be the day that I die.
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "twirl" is a verb.
The part of speech for "explicit" is an adjective.
The part of speech for "indefinite" is an adjective.
The part of speech for "lecturer" is a noun.