Chief saw a kind of green mucous (5)
I believe the answer is:
slime
'chief saw' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'a kind of green mucous' is the wordplay.
I cannot really understand how this works, but
'green' could be 'lime' (lime is a shade of green) and 'lime' is located in the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for slime that I've seen before include "Goo" , "Gunk" , "Unpleasantly moist and slippery substance" , "Nasty slippery stuff" , "Gunge" .)