Camilla's back in the king's coach (3)
I believe the answer is:
car
'coach' is the definition.
(as in a train coach or car)
'camilla's back in the king's' is the wordplay.
'back' says to take the final letters (the letter at the back).
'in the' indicates putting letters inside.
'king' becomes 'cr' (short for Carolus Rex, King Charles in Latin).
The final letter of 'camilla' is 'a'.
'a' put inside 'cr' is 'CAR'.
(Other definitions for car that I've seen before include "Private vehicle" , "Road vehicle; rail coach" , "4-wheeled vehicle" , "Automobile" , "saloon perhaps" .)