Clear definitions for symbols and functions

f[x_] := {x, x}

? f

InformationData[<|ObjectType -> Symbol, Usage -> Global`f, Documentation -> None, OwnValues -> None, UpValues -> None, DownValues -> Information`InformationValueForm[DownValues, f, {f[x_] :> {x, x}}], SubValues -> None, DefaultValues -> None, NValues -> None, FormatValues -> None, Options -> None, Attributes -> {}, FullName -> Global`f|>]

Clear[f]

? f

Missing[UnknownSymbol, f]

x = 2

2

Clear[x]

? x

Missing[UnknownSymbol, x]

Remove[x]

? x

Missing[UnknownSymbol, x]