Adamant (8)
I believe the answer is:
resolute
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">adamant</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for resolute that I've seen before include "Determined or decided" , "'Determined, dead set on (8)'" , "Determined, unwavering" , "in a determined manner" , "'With fixed purpose, steadfast (8)'" .)