"During the product realization, what requirement applies for a component for design and development (Clause 8.3). This component will be a part of an assembly, however this component can also be sold an individual component. The organization conducts design and development for an assembly however it does not conduct any D&D for a component. Can I exclude the design and development requirement for a component here?"
If the component is manufactured by your company it must be designed and the production process developed. If your company only assembles it must design and develop the assembled product but not the individual components. For example, a company can manufacture an industrial equipment. Design and development applies to the equipment as an whole and to some parts developed inside the company. The equipment incorporates a standard motor that the company buys to an approved supplier. The company do not need to design and develop that motor. And if during use, due to a breakdown of the equipment, the customer t hat bought the equipment orders a new motor, it is just a commercial operation included in the after-sales service. So, in that case you can exlude design and development for those standard components.