Toxin in river found by one boy (6)
I believe the answer is:
poison
'toxin' is the definition.
(I've seen this before)
'river found by one boy' is the wordplay.
I cannot quite understand how this works, but
'river' could be 'po' (Italian river) and 'po' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'boy' could be 'son' (synonyms) and 'son' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for poison that I've seen before include "Meat for someone else" , "Dangerous substance" , "Bane" , "[ADDER] 's defence" , "Venom, toxin" .)