The speaker is a congressman who sounds oddly eager to be dressed for his own funeral. He asks to be fitted for a black suit, and the suit is "full of yesterday's pleasure," as if the comforts of the past are what he is now wearing, or what he has already used up. Black suggests mourning, but it is also the colour of formal office, so the two ideas blur together.
The second half sums up a career. He will "do business with loss and war," which makes human suffering sound like routine paperwork. He plans to keep at it until his eyes fail, and then comes the grim final image: he will bring "a rat / home to supper." The poem suggests that after a lifetime of this work, he will lose his judgement, or his standards, so completely that he accepts something filthy as a meal. The mood is dry, bitter and darkly funny, with a sense of moral rot settling in quietly.