Corpses rotten fellow declares (8)
I believe the answer is:
cadavers
'corpses' is the definition.
('cadaver' can be a synonym of 'corpse')
'rotten fellow declares' is the wordplay.
'rotten fellow' becomes 'cad' (a cad is a dishonourable person).
'declares' becomes 'avers' (averring is a kind of declaring).
'cad'+'avers'='CADAVERS'
(Other definitions for cadavers that I've seen before include "remains" , "Dead bodies of human beings" , "Carved as corpses may be" , "Dead bodies, corpses" .)