The poem begins with the phrase "I imagine," establishing a first-person perspective and taking the reader straight into the speaker's mind. The scene is dark and quiet, and the speaker seems to be in a room with a clock and, perhaps, a desk (given that his fingers "move" across a "blank page"—implying that he's preparing to write).
It's late at night—midnight, to be precise. Time works in mysterious ways in the poem, starting right here: a single "moment" possesses, or creates, a forest (it's "midnight's forest"). There might be an actual forest outside the speaker's window, or this might be a forest of the imagination, a metaphor for the mysterious, untamed world of the speaker's unconscious mind. In any case, the alliteration and consonance—two distinctly poetic devices—between "midnight" and "moment" add lyricism to the line itself.
The speaker feels that "something else is alive / Besides" the clock and the blank page. "Besides" here could simply be mean next to—as in, there's another presence spatially near the clock and page—or apart from—as in, there's another living presence that's not the clock or the page. This second reader suggests that the clock and page are themselves alive (perhaps this is in the sense that they're alive with potential). Personifying the clock as lonely also implies that there's no other sound apart from its ticking, and makes the speaker seem all the more isolated.
Again, though, that the speaker isn't the only living thing around. The stage is set for "something else" to come into being—but the nature of that "something" is intentionally left unspoken at this early stage in the poem.
It's worth noting how the use of end-stop and enjambment play with the reader's experience of time in the poem so far. The end-stop after line 1 (that colon after "forest") creates a definite pause, and suggests that everything that follows comes from the speaker's mind, that it's all part of his imagining.
This end-stopped line then contrasts with the dynamic movement of the three lines that follow. Lines 2-3 ("Something else [...] loneliness") flow quickly down the page until firmly stopping, somewhat ironically, after the word "move" in line 4, coming rest on the image of the blank page. It's as though the speaker can feel creative inspiration stirring but can't yet harness it. This rhythm also anticipates the stop-start movements of the fox so beautifully described in the next stanza—and the tension between inaction and action of a poet waiting patiently for poetry to happen.