Fertilizer compound (4)
I believe the answer is:
urea
'fertilizer compound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for urea that I've seen before include "Waste product of mammals" , "Human waste product" , "Waste material" , "Waste matter" , "Compound found in urine, synthesised as a fertiliser" .)