Unfinished song, by the way, is heavy stuff (7)
I believe the answer is:
ballast
'heavy stuff' is the definition.
(I've seen this before)
'unfinished song by the way' is the wordplay.
'unfinished song' becomes 'balla' (I can't explain this - if you can you should give a lot more credence to this answer).
'by the way' becomes 'st' (I've seen this in other clues).
'balla'+'st'='BALLAST'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for ballast that I've seen before include "Heavy material taken aboard to steady ship" , "Heavy material giving ship stability" , "Material put into a ship to improve its stability" , "Mass giving ship stability" , "Material used to stabilize a ship" .)