JTS: RID Lookup is used to retrieve information from the table that is not part of the index. For example lets say you have a table with col1, col2, col3, col4. You have a non-clustered index on col1. You execute "Select * from Table1 Where Col1 = 123"; system will do a quick seek on col1 to get RID information for all the values in col1 where it is 123. Then using the RID information it will go to the heal to get the remaining information requested by the query, namely col2, col3, and col4. RID tells SQL Server where the row is physically located and for a singleton requests is fairly quick. However if you have scans or range retrieval of the information it can get expensive :).

Answer you question, should you add Clustered Index? Since this is a 3rd party application most likely not a good idea as most 3rd party vendors will void warranty if people change their database or application. So please take that into consideration. However define large table for me? If the table has more then 1000 pages, maybe look at considering a cluster index. Alternative you might be able to convert one of your non-clusters indexes to clustered. Test, test, and test... before you implement anything :).

Cheers!