Make an oopsie (4)
(Other definitions for slip that I've seen before include "Lingerie item - fielding position" , "Glide; petticoat" , "Strip" , "Minor mistake - undergarment" , "potter has to cover?" .)
'make an oopsie' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for err that I've seen before include "Make mistake in Canberra" , "Go astray" , "Drop a clanger" , "Miss the mark" , "Leave the straight and narrow" .)