It was an AU where Tony pretty much adopted Clint as his brother, and Clint becomes involved with SHIELD as a kind of babysitter to the children of other comic characters, while the rest of the Avengers do their Avengers thing. It was also Clint/Coulson but that was definitely an aside, focusing more on Tony and Clint's friendship.
I'm fairly certain it was called 'Not For Lack of Trying' and used to be archived on AO3, but all of my bookmarks come up as blank. If anyone knows a working link/PDF, I would be so grateful! Thank you so much for any help! c: