The clutches are in line in the center of the transmission, and are applied by pressurized transmission fluid through passages that go through the valve body, the transmission housing, the front pump and then to the clutch drums. (There are other designs that have center housing supports that also serve to provide fluid flow.) If you have a clutch that is slipping, it can be caused by a loss of pressure. The pressure loss could be a filter, or pump problem (unlikely). Or it could be a loss of pressure through any of the seals that direct the fluid flow to the clutch drum, bad clutch piston to drum seals (common), or of course simply worn out clutches.

Its likely you will need an expert to rebuild the transmission.

You asked if the "clutches" were adjustable, they are not. Now most transmissions that use bands, can have adjustments for the bands, and they can indeed be external and should be checked during a transmission service. BUT, once the transmission is already slipping, its too late for an adjustment to fix that. Besides, a band problem will make the transmission slip in a specific gear when its needed to be applied, it is not used with any of the others. Since you didn't specify that the problem is occurring in a single gear range such as " it only slips in second gear" you did not appear to be asking about band adjustments.

Maybe some of the others that read this site will tell you who has the knowledge and experience to answer your transmission questions. If so maybe that person will respond.....