I went down a bit of a rabbit hole and noticed that wiktionary.org has an API, for example:
IME their articles are written with learners in mind, too. I would probably count as a proficient English user that is still learning new words (well, who does not).
Arbitrary word for example: https://www.collinsdictionary.com/dictionary/english/gremlin
I've also found that things like thesaurus/synonym information is incomplete.
The annoying thing about Wiktionary (for this purpose) is that it's explicitly not machine readable, so the entries can only really be handled as free-form HTML. Parsing the Wikicode, despite heavy use of standard templates, would be very hard to do robustly.