Origin of heartless automaton (4)
I believe the answer is:
root
'origin' is the definition.
('root' can be a synonym of 'origin')
'heartless automaton' is the wordplay.
'heartless' suggests removing the centre.
'automaton' becomes 'robot' ('robot' can be a synonym of 'automaton').
'robot' with its middle taken out is 'ROOT'.
'of' acts as a link.
(Other definitions for root that I've seen before include "Rummage, origin" , "top of tree? Hardly!" , "grub" , "Turn up with the snout." , "Origin; eg, carrot" .)