I don't think that makes sense for an entry.

Created attachment 139419 [details]
Example

Basic concept: See widget styling as language, where always the same expressions should be used for the same things.

To say clickable apply a button-look. The entry is clickable. Button-look would feel odd without a depressed state, though.

A depressed state could be a nice touch even with a sunken look.

So I ask for a possibility for themes to do this.

Wouldn't it be more consistent for the entry to have a prelight state to indicate that it is susceptible to clicks?

A depressed ("clicked") state might indicate that the click actually triggers some action -- like a button -- rather than just moving input focus.

There should be prelight for everything click- or dragable. Which doesn't mean all themes should use it in all cases.

I guess entries seem to not have it currently, because the mouse cursor changes on hover.

The depressed state should just help to make things feel solid, more closely relating the click to the touch of a finger.

I second Rob's concern, a depressed state of a widget indicates that clicking does do something. For instance you would assume a bug if a button didn't respond to a click.
What I see on the mockup looks like a button to me, disregarding the information that it is supposed to be an entry.