Where is the fuel for army vehicle? (4)
I believe the answer is:
tank
This clue is a double definition.
'where is the fuel' is the first definition.
(as in a fuel tank)
'army vehicle?' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for tank that I've seen before include "Combat vehicle" , "Army vehicle - reservoir" , "Miltary vehicle" , "defeat" , "Military vehicle (for fish?)" .)