Need a textfield required only when selectlist is set to 'Approved' within custom content type
Field Conditional States will help you modify the look of the field: it will make it appear or disappear, or will add the red asterisks to make it look required. But as you noticed, making it look required is not the same thing as making it required. What I have done is using a combination of Field Conditional States to change the look of the field, AND Field Validation to actually make it required (so that the form returns the "Field required" error message when submitted). Field Validation has a way to return the error by checking that the field is not empty, AND checking if a different field in the form has a particular value. You need the "PHP code" type of validation for this. Take a look at the documentation for this module's PHP code option.