There is no such thing as a "sum" field. There are fields of type summary and there are fields of type calculation that use the sum() function to get totals and subtotals in FileMaker. And we can now get the same type of totals using calculation fields that use ExecuteSQL with a sum function as part of the SQL query.
As best I can determine, you have this relationship (though with your names in place of mine):
Projects::__pkProjectID = WorkLog::_fkProjectID
Summary fields and sum functions aren't going to work here without a bit of careful design work. ExecuteSQL, if you have FileMaker 12 or newer and want to tackle SQL queries, can actually be a much more straight forward approach: FMP 12 Tip: Summary Recaps (Portal Subtotals)
Using FileMaker relationships, you can link in a table of ServiceClassification Records and use a global field defined in the same table to filter this down to just the records for the current project:
Projects::anyField X ServiceClassifications::anyField
ServiceClassifications::ServiceClassification = WorkLog|ServiceClass AND
ServiceClassifications::gProjectID = WorkLog|ServiceClass::_fkProjectID
WorkLog|ServiceClass would be a new table occurrence of WorkLog and gProjectID would be a global field of the same data type as _fkProjectID.
A calculation field defined in ServiceClassifications can be defined as:
Sum ( WorkLog|ServiceClass::Actual Time )
Then you can place a portal to ServiceClassifcations on your layout and use a portal filter to omit any records where the above calculation field is zero or empty to show just the subtotals for each service Classification for the current project record.
You would set up the OnRecordLoad trigger to perform this script in order to make sure that gProjectID always has the correct value:
Set Field [ ServiceClassifications::gProjectID ; Projects::__pkProjectID ]