One who worked really hard at an evil trade? (6)

I believe the answer is:

slaver

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'one who worked really hard at an evil trade?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

This is all the clue.

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(Other definitions for slaver that I've seen before include "Poisonous vapour" , "Water running out" , "drivel" , "Dribble from the mouth" , "Vessel with cramped accommodation" .)

I've seen this clue in the Evening Standard.

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