Hey there alexhubley,
Thanks for reaching out to Apple Support Communities. I understand you’re having issues using the Thesaurus on your Mac. I’ll be happy to help with this.
You mentioned this doesn’t work in the Dictionary app, but what if you use the Look Up feature? Does that work? Take a look at this article for info on how to do this:
Look up words on Mac
If so, try using the Dictionary in safe mode and see if it works there:
How to use safe mode on your Mac
Dictionary User Guide for Mac
‘If the issue continues while in safe mode, or your Mac restarts several times and then shuts down while starting up in safe mode, you should reinstall macOS and make sure that your Apple software and third-party apps are up to date.
If the issue doesn't continue while in safe mode, leave safe mode by restarting your Mac normally. Then test for the issue again. The issue might be resolved.
If the issue returns after leaving safe mode, startup items are probably at fault. Apps can install such items in numerous places, and they aren't always easy to find or recognize. For that reason, you should first make sure that you've installed any available updates for your Apple software and third-party apps.
If updating your software doesn't help, try any of these solutions:
Regards.