Gunk or goo (5)
I believe the answer is:
slime
I believe this is a double definition.
'gunk' is the first definition.
(I know that gunk can be written as slime)
'goo' is the second definition.
(I know that goo can be written as slime)
(Other definitions for slime that I've seen before include "Nasty slippery stuff" , "Sludge, gloop" , "Goo" , "Gunk" , "Gunge" .)