I have a large POSTGIS database table with 10+ million features and I'm trying to "clip" it to another geometry that is already read into the workbench. Is there any examples of this in a tutorial or something?

Due to the volume of features you're potentially trying to clip i'd firstly look at using a FeatureReader with sptaial filtering (intersect) enabled to read in your data (use the clip geom as the initiator). This will read in all data from PostGIS that intersects the initiator geom.

Then pass the features you read in into the clippee port on a clipper, and pass the clip geom into the clipper port

http://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Transformers/Transformers/featurereader.htm

https://www.safe.com/transformers/clipper/