Tafsir of Al-Muzzammil 73:3

"(Half of it)" is a substitution (badal) for "a little" (qalilan), which is a substitution of the whole for the part (badal al-kull min al-ba'd). The pronoun refers to the night. In this substitution, there is a removal of ambiguity. Bringing the term "a little" serves to indicate that the half which is submerged in the remembrance of Allah Almighty is equivalent to the whole, while the empty half—even if equal in quantity—is not equal in reality [to the occupied half].

"Or subtract from it" is a conjunction connected to the preceding command, and the genitive pronoun refers to the night as well, qualified by the exception, because that is what the speech was directed toward. It is said that it refers to the half due to its proximity. "A little" means a small subtraction or a small amount, such that it does not descend below a quarter [half of the half].