Now we find out why the speaker was so interested in this interrupted cry, other than that it's a dark, creepy sound on a dark, creepy night. The cry was not for him, though he wished it was calling him back from wherever he came from, or yelling a forgotten goodbye.
So, the speaker has stopped to listen to this interrupted cry, which comes from far away, over houses on another street. After all that effort to stand still and stop the sound of his footsteps, it's just a random cry. It has nothing to do with him. No one wants him to stick around for the night, no one wants to tell him goodbye. It's just a lonely cry, on a lonely night.
So, now, both of the speaker's chances for human interaction have failed. He saw a watchman, but avoided him, and he heard a cry, but it wasn't for him.
And further still at an unearthly height
Now we look up to the sky, which is probably what the speaker means by "an unearthly height."
Again, we get the sense of being really far away and lonely: "further still" reminds us of the "furthest city light" and of the cry that came from "far away." But now we're looking at something that is at an unearthly height, the farthest away of all.
One luminary clock against the sky
Now we're told what's at "an unearthly height" – a luminary clock in the sky – but we're not quite sure what that is, either.
It helps to look up the word "luminary." This word is often used to describe something that is really bright or glowing, or someone who is really smart. But it also means celestial, like the sun, stars and the moon.
While this clock could just be a big clock tower in the city, the speaker tells us that this clock is luminary, and that it's at an "unearthly" height. These are big clues that this clock is not actually a clock. Since it's at night, our speaker is probably talking about the moon.
Remember, our speaker is acquainted with the night. He's probably been out here a good bit before, and is used to things like trying to tell what time it is by looking at the location of the moon in the sky.
While this clock at an unearthly height is a little spooky, it's also beautiful, and gives us light beyond the farthest city lights. The moon is distant and dreamlike.