Fuel used in older vehicles (4)
I believe the answer is:
derv
'fuel' is the definition.
(derv is a kind of fuel)
'in older vehicles' is the wordplay.
'in' says the answer is hidden in the clue.
'DERV' is hidden within 'older vehicles'.
'used' is the link.
(Other definitions for derv that I've seen before include "Type of fuel" , "Fuel for lorries" , "Water monster" , "Vehicle diesel-oil" , "Vehicle fuel oil" .)