fixer has 9 distinct synonyms spread across 4 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
someone who intervenes with authorities for a person in trouble (usually using underhand or illegal methods for a fee)
a chemical compound that sets or fixes something (as a dye or a photographic image)
a skilled worker who mends or repairs things
synthetic narcotic drug similar to morphine but less habit-forming; used in narcotic detoxification and maintenance of heroin addiction
Synonyms, antonyms and the broader and narrower relations for fixer come from Princeton University’s WordNet. Full sources and licences.