Not with the UI, but it’s possible with a little bit of scripting. Since this will affect many items, I strongly recommend making a backup of your Zotero directory before proceeding. See: https://www.zotero.org/support/zotero_data

Once your data is securely backed up, you can follow these steps to clear the value of the "extra" field in all selected items:

1. Select the items you’d like to modify (use Cmd/Ctrl+A in the root of your library to select all).
2. From the menu bar, choose Tools → Developer → Run JavaScript.
3. Tick "Run as async function"
4. Paste the following code:

let clearExtra = async (i) => {
if (i.isRegularItem()) {
i.setField("extra", "");
await i.saveTx();
}
};

await Promise.all(
ZoteroPane.getSelectedItems().map(i => clearExtra(i))
);

5. Press Run.

(I've edited the code few minutes after posting initially to make sure it correctly waits for the changes to execute)

yes, saved me a lot of time, many thanks!

Firstly, for regular fields, custom script is no longer needed. Latest beta supports batch editing. However, current implementation does not support batch-editing authors yet.

The following script will remove ALL book authors from EVERY SELECTED book section item. Same instructions as before (Run as async function).

Before applying, please see my warning about scripting and backup from the original message.

let bookSectionID = Zotero.ItemTypes.getID("bookSection");
let bookAuthorID = Zotero.CreatorTypes.getID("bookAuthor");

let clearBookAuthors = async (i) => {
if (i.itemTypeID !== bookSectionID) {
return;
}
let creators = i.getCreators().filter(c => c.creatorTypeID !== bookAuthorID);
if (creators.length === i.numCreators()) {
return;
}
i.setCreators(creators);
await i.saveTx();
};

await Promise.all(
ZoteroPane.getSelectedItems().map(i => clearBookAuthors(i))
);