Slimy substance (3)
I believe the answer is:
goo
'slimy substance' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for goo that I've seen before include "Substance that's viscous" , "Viscous matter" , "Sticky or slimy substance" , "Slime" , "Anything sticky" .)