Rapidly spreading conflagration (8)
I believe the answer is:
wildfire
'rapidly spreading conflagration' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)