The most appropriate one-word substitution for the given group of words "An act of explaining something" is "Exposition."

Explanation of the options:

☀ Invocation: This term refers to the act of invoking or calling upon a higher power or deity, typically in a religious or ceremonial context. It does not specifically capture the meaning of explaining something.

☀ Exposition: This term accurately represents the act of explaining something in a clear and detailed manner. It involves presenting information, ideas, or concepts in a systematic and coherent way.

☀ Debonair: This term describes a person who is stylish, charming, and sophisticated in appearance and behavior. It does not directly relate to the act of explaining something.

☀ Arbitrator: This term refers to a person who is appointed to settle disputes or make decisions in a fair and impartial manner. While it involves the resolution of conflicts, it is not specifically focused on explaining something.