WordNet records no synonym and no antonym for toehold. 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.
a relatively insignificant position from which future progress might be made
“American diplomacy provided a toehold on which to proceed toward peace talks”
a wrestling hold in which the toe is held and the leg is twisted against the joints
Synonyms, antonyms and the broader and narrower relations for toehold come from Princeton University’s WordNet. Full sources and licences.