Water storage used by the armoured corps (4)
I believe the answer is:
tank
This clue might be a double definition.
'water storage' is the first definition.
'tank' can be an answer for 'storage' (thesaurus). I'm not sure about the 'water' bit.
'the armoured corps' is the second definition.
'tank' can be an answer for 'corps' (thesaurus). I'm not certain of the 'the armoured' bit.
'used by' acts as a link.
Can you help me to learn more?
(Other definitions for tank that I've seen before include "Cistern - armoured vehicle" , "Water reservoir" , "Septic container" , "See 1 Across" , "War machine" .)