Is it possible to define (or assume) the derivative of a function

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I think that the short answer to your question is 'no'. However, to get the output of your example, you can still define f1 as the primitive of f2+f3:

sage: f2 = function('f2', latex_name='f_2')
sage: f3 = function('f3', latex_name='f_3')
sage: f1(x) = integrate(f2(x)+f3(x), x)
sage: diff(f1(x), x)
f2(x) + f3(x)
sage: f1.derivative(x)
x |--> f2(x) + f3(x)

You should be able to create a class of a function, though, and give it a custom derivative.

https://github.com/sagemath/sage/blob...

    # only one of derivative and tderivative should be defined
    if hasattr(self, '_derivative_') and hasattr(self, '_tderivative_'):
        raise ValueError("only one of _derivative_ or _tderivative_ should be defined.")

There are a number of examples in the src/sage/functions directory, e.g. https://github.com/sagemath/sage/blob...