The most general way to do so would be to evaluate the expression using the Compiler:
expression = element_named_type.feature_value_expression
result = syside.Compiler().evaluate(expression)[0]
The result in this case would be the element Structure::VehicleType::combi. (Note: the [0] at the end is needed because the compiler evaluation returns tuple[syside.Value | None, syside.CompilationReport])