Drop car into vessel (8)
I believe the answer is:
diminish
'drop' is the definition.
(I know that drop is a more specific form of the action decline)
'car into vessel' is the wordplay.
'car' becomes 'mini' (Mini Cooper).
'into' indicates putting letters inside.
'vessel' becomes 'dish'.
'mini' going into 'dish' is 'DIMINISH'.
(Other definitions for diminish that I've seen before include "Make smaller or less" , "sink" , "Diet" , "cut down" , "Get smaller" .)