Best-selling author a shade pointless, giant in Croesus? (7,5)

I believe the answer is:

richard osman

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'shade pointless giant in croesus?' is the definition.
The definition suggests a singular noun which matches the answer.

'best-selling author a' is the wordplay.
I cannot quite understand how this works, but
'best' could be 'a' and 'a' is present in the answer.
'a' could be 'an' and 'an' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Independent.

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