It's a vertice that's not being sufficiently included or weighted in the shoulder rig...

To fix this, open the joint editor palette...

Rotate the Up-Down (zRotate) parameter for the shoulder so that the defect is showing.

Assuming that this is a weight mapped rig, with the up down settings in the joint editor selected via the dropdown, click the brush icon...

Choose the add brush setting...

Gently stroke the vert into proper weight position...

Generally, the vert should be green when fully weighted. Once fixed close the joint editor.

If this is Poser traditional weights either scale up the falloff sphere to include the vert or increase the span of influence of the the green deformer arms to encompass the offending vert.

Sorry, perhaps there's a way to do this in the fitting room. I don't use it, I use the set up room and sculpt fits in an external sculpting app. In any case this is a job for the joint editor no matter the method.