Flash risible leader? One's into that! (6)

I believe the answer is:

mooner

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'that' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'flash risible leader? one's into' is the wordplay.
'flash' becomes 'mo' (both mean a very short period of time).
'leader?' indicates taking the first letters (I've seen 'leaders of' mean this).
'into' is an insertion indicator.
The initial letter of 'risible' is 'r'.
'mo'+'r'='mor'
'mor' placed around 'one' is 'MOONER'.

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I've seen this clue in The Sunday Times and The Times.

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