Mistreatment of older chief who'd been appointed to a position of trust (12)

I believe the answer is:

officeholder

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'who'd been appointed to a position of trust' is the definition.
I can't tell whether this definition defines the answer.

'mistreatment of older chief' is the wordplay.
'mistreatment' becomes 'of' (I can't justify this - if you can you should give a lot more credence to this answer).
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'older'+'chief'='olderchief'
'olderchief' anagrammed gives 'ficeholder'.
'of'+'ficeholder'='OFFICEHOLDER'

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I've seen this clue in The Irish Times.

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