Disease of horses (8)
I believe the answer is:
glanders
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">disease of horses</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for glanders that I've seen before include "Disease of horses" , "It could be fatal for hacks" , "Problem horse may have" .)