Yosemite or Yellowstone (4)
I believe the answer is:
park
'yosemite or yellowstone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for park that I've seen before include "South ---; TV cartoon" , "See 8" , "area of open land" , "Leave your car (among the trees?)" , "Green or Hyde" .)