An error in the field (4)
I believe the answer is:
slip
'an error' is the definition.
(I know that slip is a type of error)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for slip that I've seen before include "Lose footing as on banana skin" , "Cutting" , "printer's proof" , "Drop" , "Error; lose one's footing" .)