The answer is hidden in the letters of 'alliterative passage? (4)hidden in hidden in alliterative; as most solvers will know'
Answer: NINA | Wordplay: Message hidden in alliterative passage? (4)Hidden in hiddeN IN Alliterative; as most solvers will know, a Nina is a hidden message in a crossword grid, the name originating from the American caricaturist Al Hirschfeld, who hid his daughter’s name in his drawings