Use case
Return the sum of the time spent of the linked issues in hours.
Expression
Create and configure a Smart field using this expression in General parsing mode:
%{sum(append(fieldValue({issue.timeSpent}, linkedIssues()), [0])) / 60}
Please, note that by adding " / 8 " before the closing curly bracket, you will receive the value in working days. You can change this number depending on the definition of a working day.