To steal books is a mindless thing (5)
I believe the answer is:
robot
'a mindless thing' is the definition.
(machine without a mind)
'to steal books' is the wordplay.
'to steal' becomes 'rob' (robbing is a kind of stealing).
'books' becomes 'OT' (Old Testament).
'rob'+'ot'='ROBOT'
'is' acts as a link.
(Other definitions for robot that I've seen before include "Man-made man - man-made machine" , "Computer-controlled machine" , "Mechanism that moves automatically" , "Automated machine" , "Programmed machine" .)