Rapidly spreading conflagration (8)
I believe the answer is:
wildfire
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rapidly spreading conflagration</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for wildfire that I've seen before include "To spread like it is to spread very quickly" , "Combustible liquid, prov. fast-spreading" , "Combustible material - how news may spread?" , "It spreads fast" , "moves fast over forest" .)