Interrupt a conversation (4,2)
I believe the answer is:
butt in
'interrupt a conversation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for butt in that I've seen before include "Break into a conversation" , "Give unwanted twopenceworth, perhaps" , "Interrupt or meddle - head first?" , "Interrupt, intrude" , "Interrupt a conversation" .)