"Yesterday" or "Tomorrow" (4)
I believe the answer is:
song
'yesterday or tomorrow' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for song that I've seen before include "Piece of vocal music" , "Musical rendition" , "Piece of music, usually with words" , "Carol, for example" , "Vocal number" .)