Longtime film rating org.

MPAA

This group assigned the first G, PG, and R ratings in the late 1960s and was later succeeded by the MPA.

This clue refers to the Motion Picture Association of America, or MPAA, which has been the main organization assigning film ratings (like PG-13 or R) in the U.S. for decades. The word "longtime" hints it’s been around a while, and "film rating org." is a straightforward abbreviation for its role. "MPAA" fits as the short, well-known acronym for the group behind those familiar movie ratings.