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
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what part of speech is beneath
adverb
The part of speech for "explicit" is an adjective.
The part of speech for "lecturer" is a noun.
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The part of speech for "breezy" is an adjective.
The part of speech for "buried" is a verb.