Crossword Genius

A call note changes suddenly (3,2,4)

Ross

I believe the answer is:

all at once

'suddenly' is the definition.
(I've seen this in another clue)

'a call note changes' is the wordplay.
'changes' indicates anagramming the letters.
'call'+'note'='callnote'
'callnote' anagrammed gives 'llatonce'.
'a'+'llatonce'='ALL AT ONCE'

(Other definitions for all at once that I've seen before include "Simultaneously - immediately" , "Suddenly" , "Together" , "Without warning" , "How daffodils appeared to Wordsworth" .)

I've seen this clue in The Irish News.
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