"Gnarly!” (3)
I believe the answer is:
rad
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gnarly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for rad that I've seen before include "Unit of radiation" , "measure of dangerous activity" .)