Astronomically huge ball placed on vase (6)
I believe the answer is:
saturn
'astronomically huge ball' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'placed on vase' is the wordplay.
'placed' becomes 'sat' (sitting is a kind of placing).
'on' says to put letters next to each other.
'vase' becomes 'urn' (urn is a kind of vase).
'sat'+'urn'='SATURN'
Can you help me to learn more?
(Other definitions for saturn that I've seen before include "One of the 1 across" , "Nearest ringed planet" , "space traveller" , "Runs at (anag) - Roman god" , "Solar system planet" .)