Choosing an unfamiliar PhD topic?
What you describe is often the common case for a PhD. But, in a letter and also in real you should have some related interdisciplinary or methodological background. Starting as a complete novice a PhD is very risky. An expert you have to become during your PhD, no one is one at the beginning, this would be a pointless odyssey. But a background is necessary, otherwise everybody could start a PhD without educational requirements like a bachelor in natural sciences/engineering  when looking for a STEM PhD.