D. H. Lawrence's poem, "Piano," is about resisting nostalgia. In the first stanza, the speaker (as an adult) is listening to a woman singing. Hearing this passionate song, the speaker imagines the image of a child sitting under the piano at his mother's feet as she sings.
In the second stanza, the speaker resists this reverie.
In spite of myself, the insidious mastery of song
Betrays me back, till the heart of me weeps to belong (5-6)
In spite of the fact that he resists, the "insidious" song (still the one he is listening to as an adult) whisks him back to his childhood days. Note the difference between the song he hears as an adult ("insidious") and the glorification of the song he hears as a child, described as a "hymn" indicating a wholesome or even moral association.
The difference between the two songs is underscored again in the third stanza. The adult song is associated with "clamour" and "appassionato" which means to perform in a passionate or dramatic way. The song and memory from his childish days is one of glamour. Here, "glamour" is not what it is associated with today (fame, money, etc.). In this poem, glamour means a charm, romance, or enchanting spell.
The simple rhyme scheme aabbcould suggest the form of the hymn, insisting on the moral, ideal family on a Sunday evening. In the last stanza, the "glamour" and "cozy" memory is too much for the speaker to resist. He knows that he is being too sentimental in glorifying this memory, a stark contrast with the "clamour" of his present experience. What the speaker is trying to do is to balance his analytical abilities with his emotions. In doing so, he is negotiating what he sees as the harsh realities of adult life with the sugarcoated memories of his childhood.