Everything turns grey - a rash reaction perhaps (7)
I believe the answer is:
allergy
'a rash reaction perhaps' is the definition.
I can't judge whether this definition defines the answer.
'everything turns grey' is the wordplay.
'everything' becomes 'all' (all things).
'turns' indicates an anagram.
'grey' anagrammed gives 'ergy'.
'all'+'ergy'='ALLERGY'
Can you help me to learn more?
(Other definitions for allergy that I've seen before include "Abnormal reaction as to, say, pollen" , "Pathological aversion" , "Largely in an unfavourable reaction, maybe to pollen" , "Hypersensitivity to a substance" , "Bad immune system response" .)