Burn hydrogen in vehicle (4)
I believe the answer is:
char
'burn' is the definition.
(charring is a kind of burning)
'hydrogen in vehicle' is the wordplay.
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'in' means one lot of letters goes inside another.
'vehicle' becomes 'car' (car is a kind of vehicle).
'h' going within 'car' is 'CHAR'.
(Other definitions for char that I've seen before include "Tea - cleaning lady" , "clean > burn" , "Burn slightly" , "Make black by burning, or a small trout" , "Noiseless" .)