When it comes to expressing the idea of "fail in", there are numerous synonyms that can be used to add nuance and variety to your vocabulary. Instead of simply stating "fail in", you could employ words like "underperform", "fall short", "flounder" or "struggle". These alternatives capture different aspects of failing in a particular endeavor, highlighting the lack of success or proficiency one may exhibit. Other relevant synonyms include "miss the mark", "not succeed at", "come up short" or "falter". By incorporating these synonyms, you can enhance your writing or conversation by diversifying your language and, in turn, effectively convey the concept of failing in a more articulate manner.