In a Retrieve action, you can use the function contains in the XPath constraint as follows:
[contains(ATTRIBUTE, $StringVariable)]
You can’t use this in Filter list activity (as far as I know), where you can only check for equal values. So you might have to use a retrieve action instead of filtering an existing list.
The other option is to loop through your list and use the contains function to check each item, and then add them to a new list to return.