Four short lines, and they hold a surprising amount. The title, "Last stand," suggests a final act of defiance, and the poem gives us a figure who refuses to give way. That figure is "ill as an ash," a striking comparison. An ash tree is tall and upright, so the line pairs weakness with a kind of stubborn dignity. The word "ill" may also hint at the diseases that can threaten trees, though that's a reading of the imagery rather than anything the poem states outright.
What the speaker feels seems to be admiration, maybe mixed with sorrow. Whoever is being described is struggling, yet "continues" and stays "straight up," upright and unbowed. The last line, "in song," lifts everything. Even at the edge of collapse, there is music.
The mood is quiet but stirring: fragile and brave at once. The brevity adds to this, as if the poem itself is standing with nothing extra to lean on.