The Thames has one, so it naturally holds water (5)
I believe the answer is:
basin
'it naturally holds water' is the definition.
The definition suggests a singular noun which matches the answer.
'the thames has one so' is the wordplay.
'the thames' becomes 'bin' (I can't justify this - if you can you should believe this answer much more).
'has' is an insertion indicator.
'one so' becomes 'as' (e.g. phrases like 'not as tall' and 'not so tall').
'bin' placed around 'as' is 'BASIN'.
Can you help me to learn more?
(Other definitions for basin that I've seen before include "Region drained by a river" , "container" , "Washbowl" , "Bowl-shaped vessel" , "dock, maybe" .)