A little radio signal saying farewell (5)
I believe the answer is:
adios
'farewell' is the definition.
(adios is a kind of farewell)
'a little radio signal saying' is the wordplay.
'a little' indicates the answer is hidden within the clue (a little bit of the clue).
'ADIOS' can be found hidden inside 'radio signal saying'.
(Other definitions for adios that I've seen before include "Bye-bye (amigo)" , "Goodbye (20 across)" , "Goodbye to Spain" , "Farewell amigo!" , "See you in Ibiza" .)