I believe this is the only one on fdroid:

Video Transcoder (Video transcoding between common formats) https://f-droid.org/packages/protect.videoeditor/

its not been updated in 4 years, and its clunky, but it does work. If you absolutely have to go opensource, this is ypur best option.

Otherwise, simple gallery (pro version) has a video editor, but its propriety, so only on the play store version, not fdroid