Here is a workaround to clear the play queue - pick an item from your library and start playing it. You will get a dialog box warning you that it will clear your current play queue. I’m running Plex for Mac 1.33.0.2444-a220eae4.
I found many posts asking this question with no response. I found other posts asking about the dialog box warning. I just put the two together. It is disappointing that the UI for the player doesn’t have a Clear Queue option.