After firstly writing off Clyde & Son (8)

I believe the answer is:

secondly

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'after firstly' is the definition.
I can't judge whether this definition defines the answer.

'off clyde son' is the wordplay.
'off' indicates an anagram.
'clyde'+'son'='clydeson'
'clydeson' anagrammed gives 'SECONDLY'.

'writing' is the link.

Can you help me to learn more?

(Other definitions for secondly that I've seen before include "Not in first place" , "Next after firstly" .)

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

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