Fuel oil expires with the last litre (6)
I believe the answer is:
diesel
'fuel oil' is the definition.
(I've seen this before)
'expires with the last litre' is the wordplay.
'expires' becomes 'dies' ('die' can be a synonym of 'expire').
'with' says to put letters next to each other.
'last' indicates one should take the final letters.
'litre' becomes 'L' (abbreviation).
The final letter of 'the' is 'e'.
'dies'+'e'+'l'='DIESEL'
(Other definitions for diesel that I've seen before include "Type of fuel oil" , "Type of engine" , "Lorry fuel" , "Motor fuel" , "Fuel used by heavy vehicles" .)