If you have this relationship:
Parent----<Child (----< means "one to many" ) and your portal is a portal to Child on a Parent layout.....
List ( Child::Field ), if evaluated from the context of Parent, will list the value of field for all related records in Child.
That "context" is a drop down at the top of the calclation editor when you are setting up a calculation field. In a script step, it's the context of the current layout's table occurrence. Any conditional format or hide object when expression also uses the layout's context.
What you describe sounds like you are trying to use list from the context of Child, such as defining the field in the child table instead of the parent.
Is that calculation field on the layout tall enough to display the resulting (CR-delimited) list? How *do* you want to display the result?
The problem with your problem is that you embed your solution into the solution.
Moreover, there's no such thing as "related fields in a portal". We can nod to "related records in a portal".
Usually, one should articulate
1) - I want this
2) - I tried this
3) - 2) Does not work, that's why I'm posting, goal is 1)
4) - Help me achieve 1), it's my primary goal
5) - Special Bonus: tell me why 2) does not work
usually you get a truckload of valid suggestions for 4)