What might be a short commentary on différance in Derrida's deconstruction?
A commentary on différance might describe what différance means and what the significance of différance is in meaning, philosophy, and literature.
Différance is a combination of two meanings "to differ" and "to defer." Derrida describes meaning as "always already deferred." A word's (signifier's) meaning is always deferred to other different words (signifiers). For example, a bear is an animal that lives in the forest, eats fish etc. A bear is not a beer or a brier, and so on. The word "bear" (and the concept or thing bear) is marked by its difference from other words. Therefore the word and concept (signified idea) is defined by, and therefore defers to, other words. Theoretically, this deferral from bear to other words could go on indefinitely. One could go on forever saying a bear does not have wings, the word is not spelled with a "Q" and so on, forever. Considering all of this, Derrida would say that the bear (or any word and/or concept) is marked by absence. By itself, the word bear" does not tell us what it is. We need at least some of those other words.
So, the presence (full meaning of the bear) is always incomplete. The presence is more like a trace linking to other words/ideas (or signifiers/signifieds to use the terminology of semiotics).
Consider this in terms of literature. The meaning of a poem is not completely contained within that poem. It is not contained within the poem and the author's intent. The meaning defers to other meanings, not just from the author's background and historical context; but from the play of signification in language itself, the poem lends itself to multiple interpretations and connections. This is especially apparent when we take a reader's interpretation into consideration. This is not to say that the reader opens up meaning to new possibilities. The reader does have the freedom to interpret but Derrida would say that the play of language and the "movement" of différance is what makes the indeterminacy of meaning possible.
One of the ways to see this is through intertextuality. T. S. Eliot's "The Wasteland" contains dozens of literary references. Therefore, the meaning extends (defers) to these (different) literary texts. We might start with "The Wasteland" but since the meaning (via différance) is not inherently centered within that poem, we could say that the meaning is decentered because it defers to these other texts. Part III of "The Wasteland" is titled "The Fire Sermon." This refers to a sermon from Buddha which could be related to the Biblical "Sermon on the Mount" which could defer to a Christian belief system which could defer to . . . etc.
This idea of decentering via différance has been used to decenter so-called complete meanings in a variety of fields. Derrida highlights binary oppositions and how they structure our thinking. One can clearly see how concepts of good have often been defined by their deferral to (opposition to) concepts of bad or evil. Therefore, each term defers and refers to the other. Good is marked by the absence of bad. To know "good" is to know "bad" and vice versa.