The term "chief constituent" refers to the main or primary component of something. Antonyms of this phrase could include subordinate constituent, minor component, or negligible element. These terms suggest that there are other parts or components that are less important than the chief constituent. Other antonyms may include peripheral element or accessory part, which both imply that the component in question is not essential to the overall function or structure of the thing being described. Additionally, the antonym minority element could be used to suggest that the chief constituent is merely one of many important components, rather than the most important.