Climber starting off in vertiginous Yosemite (3)
I believe the answer is:
ivy
'climber' is the definition.
(ivy is a climbing plant)
'starting off in vertiginous yosemite' is the wordplay.
'starting off' says to take the initial letters.
The initial letters of 'in vertiginous yosemite' is 'IVY'.
(Other definitions for ivy that I've seen before include "Climbing-plant" , "The ... League comprises USA's top universities" , "Song: 'Poison --'" , "With holly, it's seasonally decorative" , "Bush" .)