Fundamental nature (7)
I believe the answer is:
essence
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fundamental nature</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 essence that I've seen before include "The most distinctive element" , "Perfume or concentrated extract" , "Distillate; gist" , "Concentrated liquid flavouring" , "Attar" .)