Mythical multiheaded beast (5)
I believe the answer is:
hydra
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mythical multiheaded beast</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for hydra that I've seen before include "that regenerated after cuts at the top" , "Mythic monster with nine heads" , "Greek island" , "Mythological water-monster" , "Proliferating-head monster" .)