Federal investigators over the pond uncovered something in the water (4)
I believe the answer is:
urea
'water' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for urea that I've seen before include "Compound found in urine, synthesised as a fertiliser" , "organic compound" , "wee substance?" , "Waste matter" , "Waste product of mammals" .)