Screenings of birthmarks essentially down by 88% (6)
I believe the answer is:
movies
'screenings' is the definition.
(film screenings)
I can't explain the remainder of the clue.
(Other definitions for movies that I've seen before include "Films in America" , "Cinema films" , "Flicks" , "big-budget features" , "Informally, films shown in cinemas" .)