Obviously, people wear different clothing with different fittings. We cannot necessarily expect everyone to wear clothing with perfect fitting. However, as a photographer, you have a responsibility to make the subject look better. In order to assure a better appearance, you should rectify the potential errors of the clothing.
As a responsible photographer, you should carry accessories like safety pins, a role of gaffers’ tape and hair ties. Since you are shooting from the front of the portrait (in most of the cases), you adjust the clothing from behind.
You can try the following methods;
From the behind, you can pull the shirt in and make it tighter. Then you can tie it to stay in place.
Pull the shirt towards the back and tuck in.
Use safety pins to hold the pulled shirt.
If not, you may pull the shirt and hold it in place using tapes.
Depending on the clothing the subject wears, you can think of different ways to adjust them.