Decline course, including car (8)
I believe the answer is:
diminish
'decline' is the definition.
(I know that decline is a more specific form of diminish)
'course including car' is the wordplay.
'course' becomes 'dish' (dish can mean a course of a meal).
'including' is an insertion indicator.
'car' becomes 'mini' (Mini Cooper).
'dish' placed around 'mini' is 'DIMINISH'.
(Other definitions for diminish that I've seen before include "Drop" , "cut down" , "Grow smaller, shrink" , "get shorter" , "Get smaller" .)