One royal in a state in Germany originally wanting this vehicle feature (3,12)
I believe the answer is:
air conditioning
'germany originally wanting this vehicle feature' is the definition.
The definition suggests a singular noun which matches the answer.
'one royal in a state in' is the wordplay.
'one royal in' becomes 'airing' (I can't explain this - if you can you should believe this answer much more).
'a state' becomes 'condition' (condition is a kind of state).
'in' is an insertion indicator.
'airing' enclosing 'condition' is 'AIR-CONDITIONING'.
Can you help me to learn more?
(Other definitions for air conditioning that I've seen before include "Publicity about health" , "Environmental control system" , "regulation of atmosphere" , "Restaurant may have it" .)