Corpse in one rotten state (7)
I believe the answer is:
cadaver
'corpse' is the definition.
(synonyms)
'one rotten state' is the wordplay.
'one rotten' becomes 'cad' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'state' becomes 'aver' (both can mean to declare).
'cad'+'aver'='CADAVER'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for cadaver that I've seen before include "Dead body, corpse" , "A kind of corpse" , "Remains of a human" , "Dead human body" , "Human corpse" .)