The speaker is on retreat, a time meant for quiet, yet the mind won't settle. The poem opens with an unsettling bit of lore: serpents in waste places feed on larks. Something dark preys on something that sings. That image sets up the tension underneath everything else, since beauty and voice seem vulnerable here.
Then the scene shifts inward. Fiddlers wander up and down the fields "in my mind," a picture of restless tunes and distractions that won't stop moving. The speaker asks how to put things in order, and calls the mind a "prison for ill words." That phrase suggests bad language, perhaps harsh thoughts or things said or half-said, is locked in with them. The final line, "I shall scour it," is short and firm. It sounds like a decision to clean out the cell, though the poem doesn't say whether it will work. The mood is tense, uneasy, and quietly determined.