Unauthorised passenger in Sierra: confiscate vehicle! (8)
I believe the answer is:
stowaway
'unauthorised passenger' is the definition.
(eg on a ship)
'sierra confiscate vehicle' is the wordplay.
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'confiscate vehicle' becomes 'tow away' (as in towing a vehicle away).
's'+'towaway'='STOWAWAY'
'in' acts as a link.
(Other definitions for stowaway that I've seen before include "One who takes a sea trip without paying" , "Ship's non-fare-paying passenger" , "Free rider" , "Non-paying passenger" , "Hider on board" .)