Poisonous bacteria girl found on fish (10)
I believe the answer is:
salmonella
'poisonous bacteria' is the definition.
(bacteria causing food poisoning)
'girl found on fish' is the wordplay.
'girl' becomes 'Ella'.
'found on' means one lot of letters go next to another.
'fish' becomes 'salmon' (salmon is a kind of fish).
'ella' after 'salmon' is 'SALMONELLA'.
(Other definitions for salmonella that I've seen before include "Bacteria causing food poisoning" , "Source of complaint" , "Bacteria associated with food poisoning" , "poisoner" , "ill-feeling" .)