WordNet records no synonym and no antonym for combination in restraint of trade. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(law) any monopoly or contract or combination or conspiracy intended to restrain commerce (which are illegal according to antitrust laws of the United States)
Synonyms, antonyms and the broader and narrower relations for combination in restraint of trade come from Princeton University’s WordNet. Full sources and licences.