Live way out, across sierra (5)
I believe the answer is:
exist
'live' is the definition.
('exist' can be a synonym of 'live')
'way out across sierra' is the wordplay.
'way out' becomes 'exit' ('exit' can be a synonym of 'way out').
'across' means one lot of letters goes inside another.
'sierra' becomes 's' (phonetic alphabet: alpha, bravo, charlie etc.).
'exit' going around 's' is 'EXIST'.
(Other definitions for exist that I've seen before include "Subsist" , "Survive" , "Scrape by" , "Have being or continue living" , "Have objective reality" .)