Fish found by girl may be cause of food poisoning (10)
I believe the answer is:
salmonella
'be cause of food poisoning' is the definition.
(I've seen this before)
'fish found by girl' is the wordplay.
'fish' becomes 'salmon' (salmon is a kind of fish).
'found by girl' becomes 'ella' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'salmon'+'ella'='SALMONELLA'
'may' is the link.
Can you help me to learn more?
(Other definitions for salmonella that I've seen before include "Bacteria causing food poisoning" , "Source of complaint" , "Nasty bug" , "Food poisoning bacterium" , "poisoner" .)