Some older vehicles use it (4)
I believe the answer is:
derv
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'some older vehicles use' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'DERV' is hidden within 'older vehicles use'.
Can you help me to learn more?
(Other definitions for derv that I've seen before include "Diesel engine fuel oil" , "Fuel for lorries" , "Vehicle fuel oil" , "Type of fuel" , "Vehicle diesel-oil" .)