Fuel in front of vehicle flaming up (4)
I believe the answer is:
derv
'fuel' is the definition.
(derv is a kind of fuel)
'front of vehicle flaming up' is the wordplay.
'front of' indicates taking the first letters (the letter at the front of the word).
'flaming' becomes 'red' (I can't explain this - if you can you should believe this answer much more).
'up' says the letters should be written backwards (in down clue: letters go upwards).
The first letter of 'vehicle' is 'v'.
'v'+'red'='vred'
'vred' back-to-front is 'DERV'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for derv that I've seen before include "Water monster" , "Lorry fuel" , "Type of fuel" , "Vehicle diesel-oil" , "Fuel for lorries" .)