Wireless announcement in port (5)
I believe the answer is:
radio
'wireless' is the definition.
(synonyms)
'announcement in port' is the wordplay.
'announcement' becomes 'ad' (short for advert).
'in' means one lot of letters goes inside another.
'port' becomes 'rio' (Rio de Janeiro is a port).
'ad' inserted into 'rio' is 'RADIO'.
(Other definitions for radio that I've seen before include "Receiving set" , "Transistor" , "Communicate with" , "Communications receiver" , "Electronic receiver" .)