When that is impossible, you should be extremely careful about the quantity and substance of what you borrow. There can be no doubt that the beating of Reginald Denny was newsworthy, but even 30 seconds was too much in KCAL's case. The fact that the material is newsworthy is a factor in favor of fair use, but not determinative if some value flows to the broadcaster at the expense of the copyright holder. And it seems that judges believe such uses can have a commercial purpose in news programs, if only a ratings increase. Finally, remember that while your station may have licensing agreements for movie clips, programs or advertisements, those agreements may not cover all uses of that material by your station.
Decisions involving fair use are never easy, even for judges.
It is very difficult to apply the four fair-use factors discussed above to the specific facts of any situation. To protect yourself, seek permission.