Stop the car starting with ease (5)
I believe the answer is:
cease
'stop' is the definition.
('cease' can be a synonym of 'stop')
'the car starting with ease' is the wordplay.
'starting' indicates taking the first letters.
'with' means one lot of letters go next to another.
The first letter of 'car' is 'c'.
'c'+'ease'='CEASE'
(Other definitions for cease that I've seen before include "Stop, finish" , "Come to an end" , "Discontinue" , "Culminate" , "Finish, end" .)