One to transmit commercial in Brazilian city (5)
I believe the answer is:
radio
'one to transmit' is the definition.
(thesaurus)
'commercial in brazilian city' is the wordplay.
'commercial' becomes 'ad'.
'in' is an insertion indicator.
'brazilian city' becomes 'rio' (I've seen this before).
'ad' placed into 'rio' is 'RADIO'.
(Other definitions for radio that I've seen before include "Broadcast medium" , "It was first called the wireless" , "listening device" , "Communications medium" , "Broadcasting medium" .)