Vault, source of water (6)
I believe the answer is:
spring
I believe this is a double definition.
'vault' is the first definition.
(I've seen this before)
'source of water' is the second definition.
(I've seen this before)
(Other definitions for spring that I've seen before include "Well - bound" , "Leap, well" , "Leap; season" , "Source of water" , "in which vegetation begins to appear" .)