Spontaneously announce that it's not from someone else's flyer (3,4,3,3)

I believe the answer is:

off your own bat

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'spontaneously announce that it's not' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'from someone else's flyer' is the wordplay.
'from' becomes 'off' (I've seen this in other clues).
'someone else' becomes 'yourown' (I can't justify this - if you can you should believe this answer much more).
'flyer' becomes 'bat' (something which flies).
'off'+'yourown'+'bat'='OFF YOUR OWN BAT'

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

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