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' says the answer is hidden in the clue.
'SET' is hidden in the letters of 'morse transmission'.
'used' acts as a 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" .)