Half of Yosemite (4)
I believe the answer is:
semi
'half' is the definition.
(I've seen this before)
'of yosemite' is the wordplay.
'of' indicates the answer is hidden within the clue.
'SEMI' is hidden within 'yosemite'.
(Other definitions for semi that I've seen before include "Not detached house" , "The next to final round" , "House; match" , "important match" , "Style of house" .)