He's learned a vehicle's parked in street (6)
I believe the answer is:
'he's learned' is the definition.
(an extremely clever or learned person)
'a vehicle's parked in street' is the wordplay.
'vehicle' becomes 'van' (van is a kind of vehicle).
'parked in' is an insertion indicator.
'street' becomes 'st' (abbreviation).
'avan' placed inside 'st' is 'SAVANT'.
(Other definitions for savant that I've seen before include "A learned person, from French" , "A learned or wise person from France" , "Intellectual" , "One of great learning" , "Man of great learning" .)