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: