Does "clear data" of a running app also "force stop"?
Yes, you are correct.  Any running app will be stopped if you clear its data, which you can verify pretty easily by selecting an app from the Running list in Settings &rarr; Apps and clearing its data; it will disappear from the list.  I verified this myself quite a while ago out of curiosity.  It is, of course, expected for the reason you mention &mdash; getting rid of the data that the app was working with will almost certainly make its state invalid or could cause partial (i.e., corrupt) data to be saved later.