Step 1 - you need to have 'Report Designer Module' > Click Design.


Step 2 - Click Field Selection > Expand QNEObjectDataSource > Expand Rows > Expand PayslipItems > Click add Calculated Fields.


Step 3 - 1st Click Edit > 2nd Insert a name for this field > Click '...' for insert Expression.


Step 4 - Paste expression 


Replace( [ItemName] ,'Allowance - ', '') 


then Click Save.

Step 5 - Drag the field to the format 


Result: