I assume you’ve tried “Reduce Noise: HQDN3D” and “Reduce Noise: Quantization” already. The last resort is “Reduce Noise: Wavelet” which can do almost magical things with low-light footage. But it is very slow. It may be better to process that one clip, save it as a new cleaned-up clip, and bring that into your final project. That way, Wavelet doesn’t have to run multiple times if you export multiple times.