Automaton died in decay (5)
I believe the answer is:
robot
'automaton' is the definition.
(I know that automaton can be written as robot)
'died in decay' is the wordplay.
'died' becomes 'ob'.
'in' is an insertion indicator.
'decay' becomes 'rot' (rot can mean to decay).
'ob' put into 'rot' is 'ROBOT'.
(Other definitions for robot that I've seen before include "Mechanism that moves automatically" , "Soulless worker" , "Humanoid machine" , "Programmable machine" , "Mechanical worker" .)