Some yeti seen roaming mountain valley (8)
I believe the answer is:
yosemite
'mountain valley' is the definition.
'some yeti seen roaming' is the wordplay.
'seen roaming' indicates anagramming the letters.
'some'+'yeti'='someyeti'
'someyeti' with letters rearranged gives 'YOSEMITE'.
(Other definitions for yosemite that I've seen before include "California falls, valley, park" , "Californian national park" , "Natl. Park in California" , "Where there are watery cascades" , "Area of beauty" .)