Mucky stuff (3)
I believe the answer is:
goo
'mucky stuff' is the definition.
(slimy or mucky substance)
This is all the clue.
(Other definitions for goo that I've seen before include "Sticky gunge" , "Anything sticky" , "Substance that's viscous" , "Sentimentality (sl.)" , "Sticky substance (colloq.)" .)