Radio used in Morse transmission (3)
I believe the answer is:
set
'radio' is the definition.
(I've seen this in other clues)
'in morse transmission' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SET' is hidden within 'morse transmission'.
'used' is the link.
(Other definitions for set that I've seen before include "Sink below the horizon" , "decline" , "Prescribed" , "(Of the sun) drop below the horizon" , "Fix; lay" .)