Product of infected tissue (3)
I believe the answer is:
pus
'product of infected tissue' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pus that I've seen before include "Yellow matter formed in infected wound" , "Product of infected tissue" , "Fluid, a product of inflammation" , "Liquid produced in infected tissue" , "Material in wounds" .)