Petrol-based agent launched principally in Vietnam (6)
I believe the answer is:
napalm
'petrol' is the definition.
(napalm is a kind of petrol)
'based agent launched principally in vietnam' is the wordplay.
'based agent' becomes 'pa' (I can't justify this - if you can you should give a lot more credence to this answer).
'principally' says to take the initial letters.
'in' indicates putting letters inside.
'vietnam' becomes 'nam' (US nickname for Vietnam).
The initial letter of 'launched' is 'l'.
'pa'+'l'='pal'
'pal' put inside 'nam' is 'NAPALM'.
(Other definitions for napalm that I've seen before include "Incendiary substance" , "Flame thrower ingredient" , "Does Ma plan to use the sticky jelly in fire bombs?" , "Liquid used in flame-throwers" , "Chemical weapon" .)