Root out a bighead - that unfeeling fellow (5)
I believe the answer is:
robot
'unfeeling fellow' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'root out a bighead' is the wordplay.
'out' indicates putting letters inside.
'a bighead' becomes 'b' (head letter of 'big').
'root' going around 'b' is 'ROBOT'.
'that' is the link.
Can you help me to learn more?
(Other definitions for robot that I've seen before include "Man-made man - man-made machine" , "Automaton or sci-fi creature" , "Soulless worker" , "Unpaid worker" , "Factory worker, perhaps" .)