Crossword Genius

Careless sixty per cent of victims (6)

Ross

I believe the answer is:

casual

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'careless' is the definition.
(similar in meaning)

I cannot understand how the rest of the clue works.

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(Other definitions for casual that I've seen before include "Free and easy" , "Informal or cursory" , "Informal, dress say" , "Type of worker employed only occasionally" , "Occasional worker" .)

I've seen this clue in The Telegraph.
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