Phlegm (5)
I believe the answer is:
mucus
'phlegm' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mucus that I've seen before include "Slimy secreted bodily fluid" , "Phlegm" , "Slimy fluid secreted in the body" , "Snot" , "Slimy secretion of nose" .)