The emotional intensity of the poem emerges in the last stanza. The rest of the poem builds up to that point.
This is a poem about poetic inspiration. The fox represents the hidden poetic muse, a wild, natural force within the speaker's psyche that is at first hidden from him.
As the poem opens, the poet is inside the house, symbolically removed from nature and his inner poetic muse, staring at a blank piece of paper as the clock ticks. In the second stanza, we see that he is alienated from his poetic self—he sees "no star" to light his way. But at the same time, amid his deep loneliness, he senses inspiration coming: "Something ... Is entering."
In stanzas three, four, and five, the speaker imagines the poetic inspiration that approaches as a fox, a wild thing. The repeated "now" in stanza three emphasizes that inspiration is arriving in the present moment. The poem moves from the clock dully ticking away time to the poet suddenly riveted by the intensity of the present moment—at this point, the emotional impact of what the speaker is feeling begins to rise.
The fox (poetic inspiration) is seen as coming in its own time and according to its own schedule. At a distance it is described with pleasant words: "bold," "deepening greenness," "brilliantly."
The poem hits an emotional crescendo in the last stanza, when the fox/inspiration suddenly strikes the speaker hard. This is not described as a pleasant experience. The fox enters the empty "hole" of the speaker's head with a "sudden sharp hot stink." This imagery reflects the emotional jolt the speaker experiences and conveys that it is more shocking and painful than delightful. Nevertheless, the poet is able, due to this pain and stink, to complete his poem.