Not quite understanding forward declaration of definitions in classes

So I'm a little confused about how this works. I've followed other examples to come up with the following:

//in entity.h
#pragma once

class Entity
{
    int nEntity;
};


//in entity.cpp
#include "entity.h"

Entity::Entity
{
    nEntity = 5;
}

and I'm not sure why the compiler doesn't like it. Any hints?