Slimy gunk (3)
I believe the answer is:
goo
'slimy gunk' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for goo that I've seen before include "Sticky substance (colloq.)" , "Substance that's viscous" , "Sticky wet material or sickly sentiment" , "mud, for example" , "Anything sticky" .)