Sexual urge (6)
I believe the answer is:
libido
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sexual urge</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 libido that I've seen before include "Sexual or psychological urge" , "Psychic, sexual, energy" , "Natural urges" , "Strong urge" , "Emotional craving" .)