Looks like possibly a gasket? That's what I would look into - unscrew that bolt in the center and see what's under the plate (or see if the bolt is loose - maybe it just needs to be tightened)
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Does this tractor have a hyd hi-lo or reverser. I think leak is between brake valve or other hyd component I mentioned of between clutch housing OR between clutch housing & the oil supply tubes(see link below).
What type hyd oil are you utilizing that is so clear in appearance??