The core of a tune (6)
I believe the answer is:
melody
'the core of a tune' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for melody that I've seen before include "Agreeable succession of musical sounds" , "My dole (anag.)" , "Musical theme" , "Sweet music, tuneful song" , "It forms the core of a tune" .)