Advertise; stopper (4)
I believe the answer is:
plug
I believe this clue is a double definition.
'advertise' is the first definition.
(I know that plug is a more specific form of the action advertise)
'stopper' is the second definition.
(I know that stopper can be written as plug)
(Other definitions for plug that I've seen before include "Cork" , "Publicise -- stop" , "Shoot" , "It fits into electric socket" , "Stopper - advertise" .)