The theme was the UK boardgame Cluedo, given by extra letters in jumbles. The thematic group and their colours comprised Miss SCARLETT (femme fatale/red), Mrs WHITE (cook/white), Mrs PEACOCK (wife/blue), Colonel MUSTARD (soldier/yellow), Professor PLUM (don/purple) and Reverend GREEN (minister/green) each to be entered in the indicated colour (not shown here). The five cells of 25ac therefore appeared blank to receive the body of Dr BLACK, forming new words. Mrs White was the murderer and words linked by secret passages (arrows) revealed location (ball/room) and murder weapon (lead/pipe, equivalent to the more exact lead piping, also accepted).
Further information can be found at www.listenercrossword.com