clinch has 4 distinct synonyms spread across 4 of its 11 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.
secure or fasten by flattening the ends of nails or bolts
“The girder was clinched into the wall”
hold a boxing opponent with one or both arms so as to prevent punches
hold in a tight grasp
“clench a steering wheel”
Synonyms, antonyms and the broader and narrower relations for clinch come from Princeton University’s WordNet. Full sources and licences.
flatten the ends (of nails and rivets)
“the nails were clinched”
(boxing) the act of one boxer holding onto the other to avoid being hit and to rest momentarily
the flattened part of a nail or bolt or rivet
a device (generally used by carpenters) that holds things firmly together