Advertise an electrical accessory (4)
I believe the answer is:
plug
I believe this is a double definition.
'advertise' is the first definition.
(I know that plug is a more specific form of the action advertise)
'an electrical accessory' is the second definition.
(Other definitions for plug that I've seen before include "Power point" , "Promote" , "Bath or sink stopper" , "Stopper - advertisement" , "Good word" .)