Hi Acer,
Have a look at the below process chain.
As shown, you should place the 'Deletion of PSA requests' component before loading your PSA. You can get this component under Other BW Processes in t-code RSPC. See below screenshot.
The specifications for this component should be as below:
I have set the 'older than days' option to 0 so that all requests older than current day will get deleted.
Hope I have answered your question.
Regards,
Jatin