Fanatic admits sell-out: “There’s nothing to it!” (4,3)

I believe the answer is:

null set

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'nothing to it' is the definition.
I can't tell whether this definition defines the answer.

'fanatic admits sell-out' is the wordplay.
'fanatic' becomes 'nut' (I've seen this before).
'admits' indicates putting letters inside.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'sell' with letters rearranged gives 'llse'.
'nut' enclosing 'llse' is 'NULL SET'.

'there's' acts as a link.

Can you help me to learn more?

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

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