Monster slain by Hercules (5)
I believe the answer is:
hydra
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">monster slain by hercules</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for hydra that I've seen before include "Multi-headed monster slain by Hercules" , "Nine-headed monster killed by Hercules" , "Many-headed snake slain by Hercules" , "Monster Hercules slew" , "Femme fatale of legend" .)