Careless sixty per cent of victims (6)
I believe the answer is:
casual
'careless' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(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" .)