A seed is treated for blight (7)
I believe the answer is:
disease
'blight' is the definition.
(thesaurus)
'a seed is treated' is the wordplay.
'treated' indicates anagramming the letters.
'a'+'seed'+'is'='aseedis'
'aseedis' is an anagram of 'DISEASE'.
'for' acts as a link.
(Other definitions for disease that I've seen before include "Illness, malady" , "scurvy?" , "Bodily disorder" , "blight" , "Contagion" .)