Hormone (9)
I believe the answer is:
adrenalin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">hormone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(adrenalin is a kind of hormone)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for adrenalin that I've seen before include "stimulant" , "Booster" , "Stimulating hormone" , "Hormone that stimulates the nervous system" , "Metabolism-affecting hormone" .)