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

If you mean add draft angle to a single part:

1- click on the part you want from the tree.

2- double click on the PartBody

3- in the command line put: c:Draft

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 ?

Hide the all parts except the part you want to do drafting in assembly
and follow the steps as shown in the tutorials