Mistake English bishop for missing head (5)
I believe the answer is:
error
'mistake' is the definition.
(I know that mistake can be written as error)
'english bishop for missing head' is the wordplay.
'english' becomes 'e' (abbreviation).
'bishop' becomes 'rr' (abbreviation for Right Reverend).
'missing head' means to remove the first letter.
'for' with its initial letter taken off is 'or'.
'e'+'rr'+'or'='ERROR'
(Other definitions for error that I've seen before include "Gaffe" , "Miscalculation" , "Faulty judgement" , "Way of operating" , "Miscarriage of justice" .)