what OOP means varies greatly by programming language. For example comparing python to clojure. The OOP part of python is that their is no equivelent to multimethod. But the languages differ in a ton of other ways. The two that stood out to me are composability and immutability. Data modeling from a functional perspective is covered in the book clojure applied. But the TLDR is that it comes down to the constraints on your problem. Persistance, performance, etc. Being dynamic, clojure tends to just mirror how your database works and doesn't require futher business domain modeling. What you have to describe are the transformations over the data.

UML is fine for what it is. Use it when you feel its worth while. Fp/clojure doesn't really impact its usefulness.