The title does a lot of the work here. A collaborator is someone who goes along with a power they may not believe in, and the poem's few lines suggest a speaker caught in exactly that position. Night is "about," meaning around and present everywhere, and in that setting the speaker feels obliged to "supply my inch." It's a small, grudging offering, as if even a tiny share of help counts as taking part.
The second stanza makes the discomfort plain. The speaker must "promise much / against my will," so the cooperation is both pressured and false. Promises are made that the speaker doesn't mean or doesn't want to make.
The poem never says what the night stands for, so it could be literal darkness, fear, or an oppressive force. Judging by the title, the last reading seems likely. The mood is tense and uneasy, quietly ashamed. The short lines and the spare wording make it feel like a confession whispered rather than spoken aloud.