Stumped by ‘Who’s Who’ crossword? Here’s the answer!

As a detective, my first step in solving the mystery of this particular crossword clue would be to focus on the keywords and context within the clue. The phrase “Who’s Who filler” suggests that we are looking for something that is commonly included in a publication or book called “Who’s Who.” The word “informally” further suggests that we are looking for a more casual or colloquial term for this particular item.

With this in mind, my attention would turn towards synonyms for filler or content that could be used to fill out a publication or book like “Who’s Who.” The word “BIOS” immediately comes to mind as a potential answer, as it is a shortened form of “biographies” – a term that is commonly used in reference to the written accounts of people’s lives and accomplishments.

As an experienced detective, I would also rely on my general knowledge and experience to confirm that “BIOS” fits the requirements of the clue. Not only is it commonly associated with filling space in publications, but it’s also a casual and widely used term for biographies in today’s digital age.

With all of these insights and thought processes in mind, I would confidently fill in the answer to the crossword clue as “BIOS“.