Wander off (5)
(Other definitions for err that I've seen before include "Slip up" , "Bungle" , "Boob" , "Make mistakes" , "Be inaccurate" .)
'wander off' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stray that I've seen before include "Lost pet" , "Go off course" , "Deviate" , "Waif and - -" , "Homeless cat" .)