Discard song, one planned as a hit? (3,5)
I believe the answer is:
lay aside
'discard' is the definition.
(thesaurus)
'song one planned as a hit?' is the wordplay.
'song' becomes 'lay' (lay is a kind of song).
'one planned as a hit?' becomes 'aside' (A-side is the main song on a vinyl single).
'lay'+'aside'='LAY ASIDE'
(Other definitions for lay aside that I've seen before include "Discard - put by for future use" , "give up!" , "reserve" , "Abandon - save for later" , "Abandon - store for future" .)