River has a chemical found in pee (4)
I believe the answer is:
urea
'chemical found in pee' is the definition.
(urea is found in urine)
'river has a' is the wordplay.
'river' becomes 'ure' (river in Yorkshire).
'has' says to put letters next to each other.
'ure'+'a'='UREA'
(Other definitions for urea that I've seen before include "wee substance?" , "It's produced by piddling" , "Waste product of mammals" , "organic compound" , "Human waste product" .)