In "A Visit From St. Nicholas," by Clement Clarke Moore, is "down" a simile for chimney in "Down the chimney St. Nicholas came with a bound"?
In "A Visit From St. Nicholas," by Clement Clarke Moore, for the line you mention, I do not believe there is a simile at all.
A simile is different from a metaphor: both compare two things that are dissimilar, though they share similar characteristics. For example, "The children swooped down on the Easter eggs like a flock of wild vultures." This is a simile, where the behavior of children and vultures is similar, though children and vultures are very different.
In the line you mention, "down" is showing the direction in which St. Nick is moving. He is traveling through the chimney, in a downward direction. There is no simile in this line at all because no two things are being compared.
As I drew in my head, and was turning around,
Down the chimney St. Nicholas came with a bound.
In this section of the poem, I do not see a simile or any other literary device: there is only end rhyme with the words "around" and "bound." One word cannot be a simile. Remember, it must be two things compared that are very different, that simply share similar characteristics. The line you reference simply means that St. Nick slid down the chimney with a lot of energy: no comparison is made here.