Chap returning holding friend's petroleum jelly (6)
I believe the answer is:
napalm
'petroleum jelly' is the definition.
(I've seen this before)
'chap returning holding friend's' is the wordplay.
'chap' becomes 'man' (I've seen this before).
'returning' says the letters should be written in reverse.
'holding' indicates putting letters inside.
'friend' becomes 'pal' (pal is a kind of friend**).
'man' written backwards gives 'nam'.
'nam' going around 'pal' is 'NAPALM'.
(Other definitions for napalm that I've seen before include "Flammable jelly used in weapons" , "Highly flammable jelly" , "Flame thrower ingredient" , "destroy by fire" , "Jelly used in fire bombs and flame-throwers" .)