This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. Usually with respect to laws or policies, in opposition to the creation or continuance of a monopoly.

n. Opposed to monopolies; the principle of opposition to monopoly.

adj. of laws and regulations; designed to protect trade and commerce from unfair business practices

"Even Hong Kong — lauded by the Heritage Foundation as the world's freest economy (de facto cartels affect the port to supermarkets to electricity to cement) — has failed to pass the kind of antimonopoly statutes that are a central pillar of developed economies around the world."

just    fair   

antitrust   

antimonopolist    antimonopolism