A fashion film for social has one job: stop the thumb. Six seconds is plenty if the first one works.
Motion in the first second
A turn, a fabric swing, a step toward camera. Static openings are skipped. Brief the film tool with the motion, not with the garment; the garment is already in the image.
One garment, one idea
Do not tour the outfit. Show the thing that makes it different: the print, the drape, the back. Everything else is in the catalog.
Vertical, then square
Produce vertical first. The square crop can be derived; the reverse loses the frame.
Sound off
Most viewers watch muted. If you add music, treat it as optional. If you add text, keep it to three words and put it where thumbs do not cover it.
Batch and test
Make five variations, post two, keep the numbers. The tool is cheap enough that testing is the strategy.