The phrase "be constituent" is often used to refer to something that is part of a larger whole. Antonyms for this phrase could include words that suggest something is not a part or component of something else. For example, "independent" suggests that something does not rely on other parts, while "disconnected" implies a lack of connection or relationship to other things. "Separate" and "isolated" may also be good antonyms for "be constituent," as they suggest a sense of distinctness or freedom from being a component of a larger whole.