Broadcast making one smile (4)
I believe the answer is:
beam
I believe this is a double definition.
'broadcast' is the first definition.
(beam can mean to broadcast or transmit)
'one smile' is the second definition.
(to beam is to smile)
'making' is the link.
(Other definitions for beam that I've seen before include "Apparatus used by women gymnasts" , "Perhaps a summer" , "Radiant smile; timber support" , "Give a wide smile" , "Ship's width; ray" .)