you can’t just delete it from the “field” – that doesn’t edit the underlying field information.
you need to “right” click on it - and edit the citation and “more” then the records cited will come up and you can click on the dropdown for the record you want to remove and “remove” it. Alternatively, you can unformat the citations in the document and work with the {Author, YEAR #RecNo} version and delete it from that then reformat them.