As a noun, clarion refers to a medieval musical instrument or the clear, shrill noise it makes. As an adjective it means loud and clear.
A clarion is a musical instrument known for making a clear, shrill sound. Its name comes from the Latin word clarus (KLAR oos) meaning clear.
The sale on winter gloves was a clarion call to me. I understood perfectly well the need to make sure my hands were fully wrapped up before temperatures dropped even further.