Review part of colonial system on those killed (5)

I believe the answer is:

slain

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'killed' is the definition.
(slaying is a kind of killing)

'review part of colonial system on' is the wordplay.
'review' is a reversal indicator.
'part of' indicates a hidden word.
'nials' is hidden in the letters of 'colonial system on'.
'nials' reversed gives 'SLAIN'.

'those' acts as a link.
This may not be correct. It may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for slain that I've seen before include "Violently killed" , "Put to the sword" , "Killed, murdered" , "Slaughtered" .)

I've seen this clue in The Hindu.

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