Seal promotion (4)
I believe the answer is:
plug
This clue is a double definition.
'seal' is the first definition.
(I've seen this before)
'promotion' is the second definition.
(plug is a kind of promotion)
(Other definitions for plug that I've seen before include "Stopper - advertise" , "quid" , "Cork" , "Piece of publicity" , "Bath or sink stopper" .)