Slimy fluid secreted in the body (5)
I believe the answer is:
mucus
'slimy fluid secreted in the body' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mucus that I've seen before include "Snot" , "Protective secretion found in the gut, nose, throat and lungs" , "Slimy secretion of nose" , "Phlegm" , "Nasal secretion" .)