What always ends with a S, and always has a C in the middle, but if you never use it then it would start with an A.
The word that always ends with an "S" and always has a "C" in the middle, but if you never use it, then it starts with an "A" is "arcs."
To arrive at this answer, we need to analyze the given information. We know that the word ends with an "S" and has a "C" in the middle, so let's focus on that. We can think of words with "C" in the middle, such as "circle," "accent," or "occurs." However, these words do not end with "S." Now, let's consider how the word would start with an "A" if we don't use it. This narrows it down further.
The word "arc" starts with an "A," but it doesn't end with an "S" or have a "C" in the middle. However, if we pluralize it to "arcs," it now satisfies all the given conditions. Thus, "arcs" is the answer to the riddle.