Chap notices alien receiver (7)
I believe the answer is:
headset
'receiver' is the definition.
(I know that headset is a type of receiver)
'chap notices alien' is the wordplay.
'chap' becomes 'he'.
'notices' becomes 'ads' (advertisements).
'alien' becomes 'et' (extraterrestrial).
'he'+'ads'+'et'='HEADSET'
(Other definitions for headset that I've seen before include "communications gear" , "listening device" , "equipment for telephonist" , "Earphones and microphone assembly" , "Communication kit" .)