Shout down (6)
I believe the answer is:
heckle
'shout down' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for heckle that I've seen before include "Shout disruptively at performer" , "Interrupt speech" , "Loudly interrupt a public meeting" , "Taunt" , "Mock or harangue speaker from crowd" .)