Hi @Anonymous User ,
1. Go to the item details of the feature service
2. Scroll to the bottom and click View for the URL:

3. Click on the layer you wish to delete features for:

4. Scroll to the bottom and choose Delete Features for Supported Operations

5. Specify 1=1 for the Where clause and click Delete Features

Another option would be to use the Delete Features tool in ArcGIS Pro. You can browse to feature services as if they were feature classes.