I have a lot of experience with aggressive fish and newcomers. Among many, here’s a few very helpful tips:
1. Introduce the damsel back at nighttime or during a water change when everything is in disarray.
2. Take the clowns out, put damsel in. Rearrange rock work, confuse the damsel, then put the clowns back.
3. Put the damsel in with any other new fish you might want. It’s not the only new guy around, so is less likely to become the boss.
4. Get other, tame damsels. But they should be faster. They’d act as dither fish/ distractions.
5. Time. Eventually, the aggression might stop. Often, a fish will beat up another only to the point where they think they’re in charge. They usually won’t kill (that is, of course, species-dependent). I’ve seen it in cichlids, and can’t imagine a damselfish being too different. Also, the clowns, as a pair, will fight back if necessary.