Transmitter essential to Brora diocese (5)
I believe the answer is:
radio
'transmitter' is the definition.
(I know that radio is a type of radio and TV emitting station)
'essential to brora diocese' is the wordplay.
'essential to' indicates the answer is hidden within the clue (I've seen this in other clues).
'RADIO' is hidden within 'brora diocese'.
(Other definitions for radio that I've seen before include "Broadcast receiver" , "Contact" , "means of communication" , "Communications medium" , "Broadcasting medium" .)