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He writes off a lot simultaneously (4,8)

Ross

I believe the answer is:

mass murderer

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'he writes off a' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'lot simultaneously' is the wordplay.
'lot' becomes 'mass' (synonyms).
'simultaneously' becomes 'murderer' (I can't explain this - if you can you should give a lot more credence to this answer).
'mass'+'murderer'='MASS MURDERER'

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I've seen this clue in the Evening Standard.
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