Something floating in the cistern (4)
I believe the answer is:
ball
'cistern' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ball that I've seen before include "20" , "Spherical plaything" , "Formal social event" , "Round object - dance" , "Spherical object" .)