Block promotion (4)
I believe the answer is:
plug
I believe this clue is a double definition.
'block' is the first definition.
(plug is a kind of block)
'promotion' is the second definition.
(plug is a kind of promotion)
(Other definitions for plug that I've seen before include "quid" , "Bath or sink stopper" , "Blatant promotion" , "Favourable mention" , "Publicise -- stop" .)