WordNet records no synonym and no antonym for shakedown. It is the only member of each of its synsets, 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.
initial adjustments to improve the functioning or the efficiency and to bring to a more satisfactory state
“the new industry's economic shakedown”
a very thorough search of a person or a place
“a shakedown by the police uncovered the drugs”
extortion of money (as by blackmail)
1 of the 4 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for shakedown.
Synonyms, antonyms and the broader and narrower relations for shakedown come from Princeton University’s WordNet. Full sources and licences.