Sinister worker kept in the dark by top assistant? (4-4)

I believe the answer is:

left-hand

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'the dark by top assistant?' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?

'sinister worker' is the wordplay.
'sinister' becomes 'left' (I've seen this before).
'worker' becomes 'hand' (hand is a kind of worker).
'left'+'hand'='LEFT-HAND'

'kept in' is the link.

Can you help me to learn more?

(Another definition for left-hand that I've seen is " Sinistral".)

I've seen this clue in The Irish Times.

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