New gallery producing bad reaction (7)
I believe the answer is:
allergy
'bad reaction' is the definition.
(allergic reaction)
'new gallery' is the wordplay.
'new' indicates anagramming the letters.
'gallery' with letters rearranged gives 'ALLERGY'.
'producing' is the link.
(Other definitions for allergy that I've seen before include "unhealthy reaction" , "Abnormal sensitivity to a food" , "Unfavourable reaction to, say, pollen" , "Hypersensitivity to a substance" , "Pathological aversion" .)