Wander off (3)
(Other definitions for err that I've seen before include "Slip up" , "Bungle" , "Boob" , "Make mistakes" , "Be inaccurate" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">wander off</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for stray that I've seen before include "Lost pet" , "Go off course" , "Deviate" , "Waif and - -" , "Homeless cat" .)