Three short lines, and a whole quiet confession. The speaker admits to a "little trouble" that makes them lie at the office. We never learn what the trouble is, and that vagueness is part of the point. It's the kind of private worry most of us carry into work and cover with a smooth face and a convenient excuse. The word "little" suggests the speaker is downplaying it, maybe to themselves as much as to anyone else.
Then the poem turns to "morning mist." It's an image, not an explanation, and it works in a couple of ways. Mist blurs edges and hides what's behind it, just as the lie does. But it's also soft and gentle, and it belongs to early autumn, a season of change. The mood is hushed and a little guilty, yet not harsh. The speaker seems more weary and rueful than ashamed, as if the fog offers some quiet sympathy for the small untruths of ordinary life.