I can't really tell anything from your pictures since there is no point of reference (side, bottom, top?). The half-moon deal looks like a pin, and the little plate is the retainer for it. If this on the bottom of the transmission, the pin is likely for the drawbar.
You will rarely ever see more than a super slow drip from a drain plug.
What you are seeing is likely not the source of the leak. Keep in mind that oil doesn't run uphill, and air movement from the fan/driving can blow fluid rearward.
There is a lot more to leak on the top and sides of the transmission than the bottom. Some tractors have a trap door in the floor pan or under the seat that will give you access to that area.
If it were mine, I would give it a pressure washing that it has never seen before, get all up in there, soaking first with a degreaser will help too. Run it a while and start following the run uphill until you find the source.
If it only leaks when the clutch is depressed, leave the tractor running and use a stick to hold the clutch down while you look.
Since you say it is related to the clutch, I would be looking the clutch operating valve over closely, if it is hydraulically applied.
I have a chased a couple on our Valtra, it is time consuming. One was a loose pipe on the top of the trans, after washing and using a really good light, you could see the fluid trail up top. The other was a loose pressure test port cap under the floor pan in the cab. I didn't find that one until I had already removed the skid plates and eliminated all of places it could leak that were down low.