Diplomat's car back shortly (8)
I believe the answer is:
minister
'diplomat's' is the definition.
(minister is a kind of diplomat)
'car back shortly' is the wordplay.
'car' becomes 'mini' (Mini Cooper**).
'back' becomes 'stern' (I've seen this before).
'shortly' means to remove the last letter.
'stern' with its last letter taken off is 'ster'.
'mini'+'ster'='MINISTER'
(Other definitions for minister that I've seen before include "Parson" , "parliamentarian" , "Clergyman - government member" , "Government or church officer" , "Cabinet member" .)