Block piece of publicity (4)
I believe the answer is:
plug
This clue is a double definition.
'block' is the first definition.
(I know that plug is a type of bung)
'piece of publicity' is the second definition.
(I've seen this before)
(Other definitions for plug that I've seen before include "See 19" , "Stop - advertising" , "quid" , "Good word" , "Stopper - advertisement" .)