Slimy secretion (5)
I believe the answer is:
'slimy secretion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mucus that I've seen before include "Slimy secretion of nose" , "Phlegm" , "Snot" , "Thick, slimy stuff" , "Nasal secretion" .)