Collie (8)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">collie</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 sheepdog that I've seen before include "could it be Old English?" , "Canine trained to control flock" , "One drives over fields" , "eg Border collie" , "Shaggy-haired canine" .)