The grey area for the clothes below is the Undershirt. You can use it to make inner clothing as the second layer.
Take Overcoat 03 for example. You can make a clothing set with shirt on the outside and undershirt on the inside.
Choose Avatar\Upper Body
Edit the Opacity texture, Cut the outer clothing open, to simulate an unbuttoned shirt
and shorten the sleeves according to your will.
You can now see inner and outer clothing.
Note: Adjusting the Brightness and Contrast of Opacity in iClone may generate abnormal results. We recommend to change those settings inside of an image editor, such as Photoshop.