In technique, I fancy, arresting copper shows incoherence (12)

I believe the answer is:

inarticulacy

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'incoherence' is the definition.
(inarticulate can mean incoherent or confusing)

'in technique i fancy arresting copper' is the wordplay.
'technique' becomes 'art' (I've seen this in other clues).
'i fancy' becomes 'ilacy' (I can't justify this - if you can you should believe this answer much more).
'arresting' means one lot of letters goes inside another.
'copper' becomes 'Cu' (chemical symbol).
'ilacy' going around 'cu' is 'iculacy'.
'in'+'art'+'iculacy'='INARTICULACY'

'shows' is the link.

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I've seen this clue in The Telegraph.

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