Origin of smelly green secretion (5)
I believe the answer is:
slime
'secretion' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'origin of smelly green' is the wordplay.
'origin of' indicates taking the first letters.
'green' becomes 'lime' (lime is a shade of green).
The first letter of 'smelly' is 's'.
's'+'lime'='SLIME'
Can you help me to learn more?
(Other definitions for slime that I've seen before include "Goo" , "Sludge, gloop" , "Gunge" , "Gooey seeping stuff" , "Moist slippery stuff" .)