Large insect (4,6)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">large insect</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 stag beetle that I've seen before include "something some folk find creepy" , "Horny little creature" , "Insect with antler-like jaws" , "Large insect with branched mandibles" , "Six-footer" .)
(Another definition for horsefly that I've seen is " sucker".)