When leader’s gone, transmits farewell (5)
I believe the answer is:
adios
'farewell' is the definition.
(adios is a kind of farewell)
'when leader's gone transmits' is the wordplay.
'when leader's gone' means to remove the first letter.
'transmits' becomes 'radios' (transmits with a radio).
'radios' with its first letter taken off is 'ADIOS'.
(Other definitions for adios that I've seen before include "Au revoir, amigo!" , "Good-bye (Sp.)" , "Bye-bye (amigo)" , "Goodbye, amigo" , "Bye!" .)