That clip is supposed to compress the O-ring. I can think of several possibilities for it not doing so.
Was the replacement O-ring as fat as the original, not just the same diameter? Soft and compressible?
Is the clip worn, such that it doesn't push the O-ring firmly in place?
Is the slot that the clip slides into worn or cracked?
Is there dirt/hair/manatee finger nails on the surface that the O-ring seats against?
Oh. If you push/hold the fitting into the side, adding pressure against the O-ring, does it then leak? If it doesn't leak, the cause is at the O-ring.
Fix it? If you can't find the problem, consider proper use of gasket
maker. Coat the surface, put the fitting into place, but don't compress it. Next day, after it sets up, compress it. People tend to use goo and immediately torque the nuts, thereby defeating the purpose of having elastic gasket
If all that fails, have a manatee sit on it.