My clothing needs a 'body'

hi! you're correct. You need to join your mesh with a nude top. But actually "joining" them in blender should be the last step.

First you need to map, weight and make both uv_0 and uv_1. Once you have those steps done, measure what parts of the body are going to be hidden behind your top and delete that extra geometry that will not be seen. Then select both groups and join them.

There's plenty of intermediate tutorials on this list that will help you out if you don't understand one of these steps.