Fundamental character (6)
I believe the answer is:
nature
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fundamental character</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 nature that I've seen before include "Disposition, temperament" , "Kind, sort; countryside" , "It abhors a vacuum, according to Aristotle" , "Essential character" , "Disposition; temper" .)