Republican Bill Moon seen in sort of telescope (5)
I believe the answer is:
radio
'sort of telescope' is the definition.
'republican bill moon' is the wordplay.
'republican' becomes 'r' (abbreviation).
'bill' becomes 'ad' (advertisement stuck on wall).
'moon' becomes 'io' (moon of Jupiter).
'r'+'ad'+'io'='RADIO'
'seen in' is the link.
(Other definitions for radio that I've seen before include "Electronic receiver" , "Transmit" , "Marconi invented it" , "Sound broadcasting" , "Transmitter" .)