Dark goo (3)
I believe the answer is:
tar
'dark goo' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tar that I've seen before include "Sticky black stuff" , "Thick flammable liquid" , "Informal sailor or coal distillation" , "Informal sailor, the rat" , "Black substance, used on roads" .)