Collision from behind upon parking (4)
I believe the answer is:
'collision' is the definition.
(I've seen this before)
'behind upon parking' is the wordplay.
'behind' becomes 'bum' (synonyms).
'upon' says to put letters next to each other.
'parking' becomes 'p' (abbreviation on road signs).
'from' acts as a link.
(Other definitions for bump that I've seen before include "Thud down, into" , "Hit" , "Impact injury" , "Slight collision" , "little impact" .)