Not entirely splendid, slimy stuff (3)
I believe the answer is:
goo
'stuff' is the definition.
(I have seen 'Sticky stuff ' mean 'goo' so perhaps 'stuff' could also mean 'goo')
'not entirely splendid slimy' is the wordplay.
I cannot really understand how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
The remaining letters 'go' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for goo that I've seen before include "Viscous matter" , "mud, for example" , "Sticky gunge" , "Anything sticky" , "Slime" .)