Burning fiercely (6)
I believe the answer is:
ablaze
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">burning fiercely</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 ablaze that I've seen before include "Having been fired" , "filled with anger" , "being toasted?" , "Fiercely aflame" , "Bright" .)