Healer Support for Horse Team

Elise is the best with Wrathful staff due to her Atk/Spd ratios overall, making her a colorless damage supply as well which Lyn now does as an archer. Clarine will be better thanks to her superior Speed, ensuring she only needs to tank 1 attack, which is safer as a support and with Fortify Cavalry applied, she can survive even more.

I had a Priscilla 5-star from her first banner and I ran her for ages, but she was super vulnerable to double attacks and was constantly dying on me.

So I'd suggest Clarine, and feed Priscilla to her for Rehabilitate and give her Imbue for enhanced healing.

Early, early on, I made the poor choice to promote Clarine to 5*. While I could've used the feathers to promote someone else, she has more than made up for it with her support/healing role on my calvary team. I gave her Fortify Calvary and never looked back. After looking at the different Stat spreads of Priscilla vs Clarine, I would pick Clarine in a heartbeat. I often use her as a buffer for one attack or to soak up a magic hit that my other units would die from. With Live to serve 2 (will probably never get 3... 5* wrys... Really?!) she can heal back her lost xp.
She also doubles a lot if you decide to go offensive for that little bit of extra damage. With Pain or Panic, having low Atk isn't always a bad thing.

All this said, it really depends on what you're using her for. If you're using her outside of Arena, Clarine is your girl. In arena though, another attacker would be more beneficial.