Commercial port screens broadcast (5)
I believe the answer is:
radio
'broadcast' is the definition.
(radio broadcast is a kind of broadcast)
'commercial port screens' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'port' becomes 'rio' (Rio de Janeiro is a port).
'screens' means one lot of letters goes inside another (screen can mean to cover up).
'ad' put into 'rio' is 'RADIO'.
(Other definitions for radio that I've seen before include "Contact" , "listening device" , "It was first called the wireless" , "means of communication" , "MW, LW or FM sound source" .)