Flickering light in slow motion?

You could try a deflicker plugin. Quick google search showed this as a possible option:

https://www.yourseedmedia.com/seed-de-flicker

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Michael Hancock
Editor

You need to change the light source, change the shutter speed on your camera, or fix it in post. Three are various flicker fixing plugins available although none of the good ones are cheap.

I usually do my grading and finishing in Resolve and that has a pretty good de-flicker plugin but for working purely writhin FCPX, seed De Flicker looks good and I've used Digital Anarchy's Deflicker plugin in the past to great effect.

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Jeff Kirkland | Video Producer & Cinematographer
Hobart, Tasmania | Twitter: @jeffkirkland

Note that NeatVideo also has deflicker capabilities now. It's worth getting for the noise reduction (which is more sophisticated than the NR built into Final Cut) and the deflicker is a bonus.

I change shutter angle from 180 to 178.2 when shooting indoors in 50hz countries (e.g., Europe) to avoid flicker; I never thought I'd need to do that in North America until I shot a concert lit with stage lights set up by the soundman...it all had terrible flicker. Resolve's deflicker removed it just fine but it's a very processor-intensive operation and I though my computer was going to launch into outer space during rendering, the fan was running so loud.