Number in an innominate (4)

I believe the answer is:

anon

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'number in an innominate' is the definition.
I can't judge whether this definition defines the answer.

'number in an' is the wordplay.
'number' becomes 'no.' (abbreviation).
'in' indicates putting letters inside.
'no' inserted into 'an' is 'ANON'.

Can you help me to learn more?

(Other definitions for anon that I've seen before include "Soon, in a little while" , "Not identified by name" , "devoid of credit" , "Soon - unknown author (abbr)" , "Without known author" .)

I've seen this clue in the Sydney Morning Herald.

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