Knock rear with pressure (4)
I believe the answer is:
bump
'knock' is the definition.
(I know that knock can be written as bump)
'rear with pressure' is the wordplay.
'rear' becomes 'bum' (synonyms).
'with' is a charade indicator (letters next to each other).
'pressure' becomes 'p' (abbreviation).
'bum'+'p'='BUMP'
(Other definitions for bump that I've seen before include "little impact" , "Speed restriction" , "Knock - collide" , "Hit" , "Light blow or jolting collision" .)