how to draft a part file alone in assembly model
Here is the tutorial showing how you can do drafting of a selected part directly from assembly.
Answered with a tutorial: https://grabcad.com/tutorials/how-to-draft-a-part-file-alone-in-assembly-model
Open an assembly, hit - Insert / new part
It will ask you for a plane to start your sketch on....
Now you will have a new virtual part, that exists within the assembly
you can open it, and choose to save it externally.
I think that is what you are asking ?