Military vehicle in reservoir (4)
I believe the answer is:
tank
I believe this clue is a double definition.
'military vehicle' is the first definition.
(tank is a kind of military vehicle)
'reservoir' is the second definition.
(source of water)
'in' is the link.
(Other definitions for tank that I've seen before include "Combat vehicle" , "Possible alternative to barrel" , "Armour-plated military vehicle" , "Storage chamber; drink heavily (colloq.)" , "Container for fish (or soldiers?)" .)