Fielder's mistake (4)
I believe the answer is:
slip
This clue is a double definition.
'fielder's' is the first definition.
(slip is a cricket fielding position)
'mistake' is the second definition.
(a slip is a blunder or mistake)
(Other definitions for slip that I've seen before include "Fall on the thinned potter's clay" , "Mistake -- small piece of paper" , "printer's proof" , "Lose footing as on banana skin" , "Slight error" .)