Incendiary weapon used extensively in the Vietnam War (6)

I believe the answer is:

napalm

(Other definitions for napalm that I've seen before include "Incendiary agent" , "Firebomb jelly" , "Petrol jelly used in flame-throwers" , "Incendiary bomb ingredient" , "Liquid used in flame-throwers" .)

I've seen this clue in The Times and The Sunday Times.

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