Five short lines capture the moment when alder trees come into leaf. The title, "Leaf-out," names that sudden change, and the poem turns it into something you can hear as much as see. The new leaves become "singing ears," as if each tree were growing a set of listening organs that also happen to make noise. They "roar and clap," which suggests wind moving through fresh foliage, or perhaps the rustle of many small leaves shaking at once.
The phrase "in disorder" gives the scene a loose, untidy energy. The alders aren't arranged neatly; they burst out in their own time and their own way. The verb "fitting themselves" makes the trees sound almost playful, like people trying on new clothes.
The speaker doesn't say how they feel outright, but the delight is easy to sense in the lively images. The mood is bright, surprising, and a little comic. Spring arrives here as noisy applause, with the woods cheering for themselves.