Ostensibly the Mechanical Turk’s Rook also retracted, pinning Bishop (5)
I believe the answer is:
robot
'ostensibly the mechanical turk's' is the definition.
(18th- and 19th-century chess-playing 'robot')
'rook also retracted pinning bishop' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'also' becomes 'too' (synonyms).
'retracted' shows that the letters should be reversed in order.
'pinning' is an insertion indicator.
'bishop' becomes 'B' (chess abbreviation).
'too' written backwards gives 'oot'.
'oot' placed around 'b' is 'obot'.
'r'+'obot'='ROBOT'
(Other definitions for robot that I've seen before include "programmed servant" , "Unpaid worker" , "Mechanism that moves automatically" , "Mechanical worker" , "Automaton or sci-fi creature" .)