Army vehicle’s to fail completely (4)
I believe the answer is:
tank
This clue is a double definition.
'army vehicle's' is the first definition.
(I've seen this in another clue)
'fail completely' is the second definition.
(I've seen this in another clue)
'to' acts as a link.
(Other definitions for tank that I've seen before include "Combat vehicle" , "Vat" , "Armoured vehicle - container for liquid or gas" , "War machine" , "place for petrol > bomb" .)