apple mail - choose sender e-mail

Just to be sure: if you have multiple accounts, the "From" field in a message being composed should be a clickable menu. But it doesn't look like a menu unless you hover your cursor over it.

Hello and welcome to Apple Support Communities, IIe.
If I understand your post correctly, you are no longer seeing the option to select which of your email addresses to send a new email from while you're composing new mail. This is an important option when you use multiple email addresses. I'd like to help.
There are several options to select which account to send new mail from:

Send new messages from

The email address to use when you send messages. If you use several email accounts, you can have Mail automatically select the best address, based on the email address of the first recipient in your message, as well as the currently selected mailbox and message.

To select these options, open Mail and click Mail > Preferences > Composing.
See the full article here:
Composing preferences
Let us know if that resolves this issue for you.
Thanks for using the Apple Support Communities. Take care.

AFAICT this menu appears if you have more than one e-mail account (I certainly see it, and use it every day, to choose between my work account, and each of my private accounts).

I don't see any way to choose among aliases for the *same* account.