Field vehicle finally takes red fuel (8)
I believe the answer is:
kerosene
'fuel' is the definition.
(kerosene is a kind of fuel)
'field vehicle finally takes red' is the wordplay.
'field' becomes 'ken' (one's field of knowledge).
'finally' suggests the final letters.
'takes' means one lot of letters goes inside another.
'red' becomes 'rose'.
The final letter of 'vehicle' is 'e'.
'ken'+'e'='kene'
'kene' enclosing 'rose' is 'KEROSENE'.
(Other definitions for kerosene that I've seen before include "Oil used as fuel in lamps and heaters" , "Combustible" , "No seeker for American fuel oil" , "Sore knee (anag.)" , "Paraffin oil (E, not I)" .)