Weapon mate used in Vietnam (6)
I believe the answer is:
napalm
'weapon' is the definition.
(I have seen 'Incendiary weapon ' mean 'napalm' so perhaps 'weapon' could also mean 'napalm')
'mate used in vietnam' is the wordplay.
'mate' becomes 'pal' (I've seen this before).
'used in' is an insertion indicator.
'vietnam' becomes 'Nam' (US nickname for Vietnam).
'pal' going inside 'nam' is 'NAPALM'.
(Other definitions for napalm that I've seen before include "Does Ma plan to use the liquid used in fire bombs?" , "Petroleum jelly used in bombs and flame-throwers" , "Liquid used in flame-throwers" , "Incendiary bomb ingredient" , "Incendiary weapon" .)