Radio transmitter (4)
I believe the answer is:
mast
'radio transmitter' is the definition.
(I know that mast is a type of radio transmitter)
This is all the clue.
(Other definitions for mast that I've seen before include "Tall radio transmitter" , "nuts and seeds" , "Upright transmitter; fruit of oak trees" , "food for pigs" , "Nuts for pigs" .)