Some yeti seen roaming mountain valley (8)
I believe the answer is:
Here is my best explanation:
'mountain valley' is the definition.
'some yeti seen roaming' is the subsidiary indication.
'seen roaming' indicates an anagram.
'someyeti' anagrammed gives 'yosemite'.
(Other definitions for yosemite that I've seen before include "US national park", "National park in California", "Californian national park", "California national park", "A US National Park", "Where there are watery cascades", "Natl. Park in California", "area", "Area of beauty" and "California falls, valley, park".)