I’ve nothing more to say about a fancy donut! (4,3,3)

I believe the answer is:

over and out

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'i've nothing more to' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'about a fancy donut' is the wordplay.
'about' becomes 'over' (both can mean around).
'fancy' indicates anagramming the letters.
'donut' is an anagram of 'ndout'.
'over'+'a'+'ndout'='OVER AND OUT'

'say' is the link.

Can you help me to learn more?

(Other definitions for over and out that I've seen before include "Radio sign-off" , "end of transmission" .)

I've seen this clue in The Times.

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