It should hold water (4)
I believe the answer is:
ewer
This might be a double definition.
'it should' is the first definition.
The definition suggests a singular noun which matches the answer.
'hold water' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
Can you help me to learn more?
(Other definitions for ewer that I've seen before include "Large water-jug" , "29 [VESSEL]" , "Large, wide-mouthed jug" , "Big jug" , "A type of jug" .)