Been away from scratch for a while.

You used to be able to right click on a block in the left pane and “define” would pop up and you could click that and the editor would go to that blocks definition. Now you only have the “edit” option.

Is there still a way to do this?

No, unfortunately. You'll have to find the definition of the custom block manually.

If you use Chrome, Griffpatch has written an Extension called “Scratch 3 Developer Tools” which has a find block function.

You can find and install the extension from the Chrome Web Store.

Thanks very much for that. The Griffpatch Extension works very nicely.