Yes, during Prohibition, it could have contained water (5)
I believe the answer is:
basin
'it could have contained water' is the definition.
The definition suggests a singular noun which matches the answer.
'yes during prohibition' is the wordplay.
'yes' becomes 'si' (I can't justify this - if you can you should believe this answer much more).
'during' means one lot of letters goes inside another.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'si' placed into 'ban' is 'BASIN'.
Can you help me to learn more?
(Other definitions for basin that I've seen before include "Sink for washing" , "Canal harbour" , "Dock - vessel" , "Water receptacle" , "Region drained by a river" .)