Under sink has cylindrical fixture instead of J-pipe, what is it and how to clear it out?
That is a drum trap. [Evidently also called a bottle trap in some parts of the world, per comments] They are rather old school, and out of favor in code now.
I'm more used to finding them embedded in a floor with the clean-out on top (for a tub application) but yours does seem to have the cleanout down.
Fair warning - they seem to be above average at &quot;wicked gross&quot; - since it's slow, not stopped, I'd run some bleach solution through (1 cup to a gallon) wait a while (if not urgent, overnight - if urgent, 15 minutes or so) then run a couple of gallons of clear water to rinse out the bleach, then put a big catch basin under it and put on gloves before opening it up.