What type of context clue is usually offset by commas within a sentence?

The type of context clue usually offset by commas within a sentence is appositive.

An appositive is a noun or noun phrase that renames or explains another noun or noun phrase. It's placed directly next to the noun it's describing and is often set off by commas.

Example:

* "My friend, a talented musician, played the piano beautifully."

In this sentence, "a talented musician" is the appositive that renames the noun "friend". It's set off by commas, providing a clear context clue for understanding the meaning of "friend".