Farewell piece about heartless Lothario (2,4)
I believe the answer is:
so long
'farewell' is the definition.
(so long is a kind of farewell)
'piece about heartless lothario' is the wordplay.
'piece' becomes 'song' (song is a kind of piece**).
'about' indicates putting letters inside.
'heartless' means to remove the middle letters.
'lothario' with its centre removed is 'lo'.
'song' placed around 'lo' is 'SO LONG'.
(Other definitions for so long that I've seen before include "Cheerio, bye-bye" , "Goodbye (colloq.)" , "That length - goodbye!" , "Au revoir" , "This extended" .)