Motor fuel going up, requiring investment of money (4)
I believe the answer is:
limo
'motor fuel' is the definition.
'limo' can be an answer for 'motor' (I have seen 'Long motor' mean 'limo' so perhaps 'motor' could also mean 'limo'). I'm not sure about the 'fuel' bit.
'going up requiring investment of money' is the wordplay.
'going up' says the letters should be written backwards (in a down clue, the letters are written upwards).
'requiring' becomes 'oil' (I can't explain this - if you can you should give a lot more credence to this answer).
'investment of' indicates putting letters inside.
'money' becomes 'm' (abbreviation used in economics).
'oil' enclosing 'm' is 'omil'.
'omil' written backwards gives 'LIMO'.
Can you help me to learn more?
(Other definitions for limo that I've seen before include "Long motor" , "Luxury car" , "Set of rooms" , "vehicle" , "Stretched car, often used for partying" .)