In South American port, notice transistor maybe (5)
I believe the answer is:
radio
'transistor maybe' is the definition.
(transistor radio)
'in south american port notice' is the wordplay.
'in' means one lot of letters goes inside another.
'south american port' becomes 'Rio' (Rio de Janeiro, port in Brazil in South America).
'notice' becomes 'ad' (both can mean an advertisement).
'rio' enclosing 'ad' is 'RADIO'.
(Other definitions for radio that I've seen before include "MW, LW or FM sound source" , "Speech receiver" , "Broadcast receiver" , "Communications receiver" , "Communications device" .)