Seat in Diana's vehicle (5)
I believe the answer is:
divan
'seat' is the definition.
(divan is a kind of seat)
'diana's vehicle' is the wordplay.
'diana' becomes 'di' (Di is short for Diana).
'vehicle' becomes 'van' (van is a kind of vehicle).
'di'+'van'='DIVAN'
'in' acts as a link.
(Other definitions for divan that I've seen before include "Sofa - bed" , "resting place" , "Couch - bed" , "Backless seat; bed" , "Long backless sofa" .)