Hypersensitivity means everything turned grey (7)
I believe the answer is:
allergy
'hypersensitivity' is the definition.
(allergy is a kind of hypersensitivity)
'everything turned grey' is the wordplay.
'everything' becomes 'all' (all things).
'turned' indicates anagramming the letters.
'grey' is an anagram of 'ergy'.
'all'+'ergy'='ALLERGY'
'means' acts as a link.
(Other definitions for allergy that I've seen before include "Cause of ill-feeling" , "Susceptibility" , "Hypersensitive reaction of the body to a foreign substance" , "Pathological aversion" , "unhealthy reaction" .)