Secret instruction that makes you cross (8)

I believe the answer is:

backdoor

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'secret instruction that' is the definition.
The definition suggests an adjective which matches the answer.

'you cross' is the wordplay.
'you' becomes 'back' (I can't justify this - if you can you should give a lot more credence to this answer).
'cross' becomes 'door' (a door is a kind of cross).
'back'+'door'='BACKDOOR'

'makes' is the link.

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I've seen this clue in The Times.

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