Interrupt (a speaker) (6)
I believe the answer is:
heckle
'interrupt a speaker' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for heckle that I've seen before include "Interrupt (a public speaker) with taunts" , "Shout abuse at, a speaker say" , "Mock or harangue speaker from crowd" , "disrupt meeting" , "Shout down (a politician)" .)