Powerless to arrest retired chap impossible to identify (9)

I believe the answer is:

unnamable

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'impossible to identify' is the definition.
I can't tell whether this defines the answer.

'powerless to arrest retired chap' is the wordplay.
'powerless' becomes 'unable' (similar in meaning).
'to arrest' is an insertion indicator (inserted letters are arrested or held).
'retired' says the letters should be written backwards.
'chap' becomes 'Man' (chap can mean a man).
'man' reversed gives 'nam'.
'unable' going around 'nam' is 'UNNAMABLE'.

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I've seen this clue in The Times.

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