It's more alarming admitting if one means to break up (9)

I believe the answer is:

scarifier

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'one means to break up' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'it's more alarming admitting if' is the wordplay.
'it's more alarming' becomes 'scarier' ('scary' can be similar in meaning to 'alarming').
'admitting' is an insertion indicator.
'scarier' placed around 'if' is 'SCARIFIER'.

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(Another definition for scarifier that I've seen is " Tool for lawn care".)

I've seen this clue in The Telegraph.

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