The poem starts with something small: an ordinary clock at school. Someone has made a great deal of it, so much that the speaker feels the truth and themselves growing "impertinent," a bit cheeky or out of line. The speaker seems to be reacting to how much weight people give to learning and expertise, and how strongly it can affect them.
The speaker still says they honor learning, so this isn't a rejection of it. Instead, they treat what they've seen as a warning to themselves: "not to master too much." Mastery has power over other people, and the speaker doesn't want to use it carelessly. The last lines turn gentle. They want to "stay with them / in their good night / as a witness," which suggests being present and quiet rather than being the authority who explains everything.
The mood is humble, thoughtful and a little tender. The "good night" could mean sleep, or something heavier, and the poem leaves that open.