Relentless; persistent
I believe the answer is:
unabated
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">relentless</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">persistent</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for unabated that I've seen before include "Undiminished" , "with no loss of intensity" , "Without diminution" , "Persistent" , "as strong as ever" .)