The phrase "We are in a sentence" can indeed serve as an introductory clause, but it depends on how it's used in a larger sentence structure. Typically, introductory clauses or phrases provide context, set the stage, or add additional information before the main clause of a sentence.
However, "We are in a sentence" itself is a complete statement and doesn't function as an introductory clause on its own. An example of how it could be used in a longer sentence with an introductory clause might be:
"Since we are in a sentence, let me explain how clauses work."
In this case, "Since we are in a sentence" acts as the introductory clause that leads into the main clause "let me explain how clauses work."