Whole green bananas can cause a bad reaction (8)
I believe the answer is:
allergen
'can cause a bad reaction' is the definition.
(allergens cause allergies)
'whole green bananas' is the wordplay.
'whole' becomes 'all' (I have seen 'The whole ' mean 'all' so perhaps 'whole' could also mean 'all').
'bananas' is an anagram indicator (informal term for mad).
'green' anagrammed gives 'ergen'.
'all'+'ergen'='ALLERGEN'
(Other definitions for allergen that I've seen before include "Protein" , "Wheat, say" , "It produces a damaging response" , "Irritant" , "Substance triggering an abnormal bodily reaction" .)