Insistent (7)
I believe the answer is:
adamant
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">insistent</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 adamant that I've seen before include "Unswerving" , "Unpersuadable" , "Refusing to change one's mind" , "Resolute; diamond (arch.)" , "Unshakable (in an opinion)" .)