The phrase "are a constituent" refers to being a part of something bigger or being an element of a particular whole. The antonyms of this phrase can be "are not a constituent," which means not being a part of something or not being an element of a particular whole. Other antonyms could include "are separate," "are distinct," or "are independent," which all suggest being isolated from something or someone. In any case, these antonyms imply disconnection or lack of involvement, unlike the idea of being a part of something bigger or being a constituent.