I say there, that was a popular song! (3)
I believe the answer is:
hey
'i say' is the definition.
Both the answer and definition are interjections.
Maybe there's an association between them I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for hey that I've seen before include "Exclamation to call attention" , "Exclamation calling attention" , "Call attracting attention" .)