Collides with a public vehicle carrying a politician (5)
I believe the answer is:
bumps
'collides with' is the definition.
'a public vehicle carrying a politician' is the wordplay.
'a public vehicle' becomes 'bus' (I've seen this before).
'carrying' is an insertion indicator.
'a politician' becomes 'mp' (Member of Parliament).
'bus' placed around 'mp' is 'BUMPS'.
(Other definitions for bumps that I've seen before include "Small protuberances; knocks lightly into" , "Jolts" , "Knocks (given on one's birthday?)" , "birthday routine" , "sleeping policemen?" .)