Prompt celebration is a central part of snooker (3,4)
I believe the answer is:
cue ball
'a central part of snooker' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'prompt celebration' is the wordplay.
'prompt' becomes 'cue' (cue can mean a prompt or signal).
'celebration' becomes 'ball' (I can't justify this - if you can you should believe this answer much more).
'cue'+'ball'='CUE-BALL'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for cue ball that I've seen before include "Feature of pub entertainment" , "Jimmy White's white?" , "Object struck by a snooker player" , "The white in snooker" .)