Crock of water topped off drain (4)
I believe the answer is:
ewer
'crock' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'water topped off drain' is the wordplay.
'water' becomes 'dew' (dew is a kind of water).
'topped' means to remove the first letter (to top something is to remove its top).
'off drain' becomes 'er' (I can't explain this - if you can you should believe this answer much more).
'dew' with its initial letter taken away is 'ew'.
'ew'+'er'='EWER'
'of' is the link.
Can you help me to learn more?
(Other definitions for ewer that I've seen before include "Jug with wide spout" , "A type of jug" , "Water jug with wide mouth" , "Open vessel with handle and spout" , "Sort of jug" .)