Something black that's so long, shortened (3)
I believe the answer is:
tar
'something black' is the definition.
(tar is a black substance)
'so long shortened' is the wordplay.
'so long' becomes 'ta-ra' (informal way of saying goodbye).
'shortened' means to remove the last letter.
'tara' with its last letter taken away is 'TAR'.
'that's' acts as a link.
(Other definitions for tar that I've seen before include "used by those who pay to work on deck" , "Thick, black and sticky liquid" , "Product of smoking cigarettes" , "Sailor (colloq.)" , "Flammable liquid; sailor" .)