Largely resolved bad reaction (7)
I believe the answer is:
allergy
'bad reaction' is the definition.
'allergy' can be an answer for 'reaction' (I've seen this before). I'm unsure of the 'bad' bit.
'largely resolved' is the wordplay.
'resolved' indicates an anagram.
'largely' is an anagram of 'ALLERGY'.
Can you help me to learn more?
(Other definitions for allergy that I've seen before include "Damaging immune response" , "Largely to do with adverse reaction" , "Susceptibility" , "Unfavourable reaction to pollen, certain foods etc" , "Antipathy (colloq.)" .)