Cheers a couple of extras (3-3)
I believe the answer is:
bye-bye
'cheers' is the definition.
('cheers' can be used to say goodbye)
'a couple of extras' is the wordplay.
'a couple of extras' means two substitutions for 'extra'.
'extra' becomes 'bye' (cricket term).
'extra' becomes 'bye' (cricket term).
'bye'+'bye'='BYE-BYE'
(Other definitions for bye-bye that I've seen before include "Au revoir for juniors?" , "Informal repetitive farewell" , "Ta-ta" , "Childish farewell" , "Ciao!" .)