Incendiary jelly (6)
I believe the answer is:
napalm
'incendiary jelly' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for napalm that I've seen before include "Incendiary liquid used in firebombs" , "Flammable jelly used in weapons" , "Firebomb jelly" , "Incendiary weapon" , "Highly incendiary liquid" .)