WordNet records no synonym and no antonym for tighten one's belt. 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.
live frugally and use less resources
“In the new economy, we all have to learn to tighten our belts”
Synonyms, antonyms and the broader and narrower relations for tighten one's belt come from Princeton University’s WordNet. Full sources and licences.