Where arm found hidden (2,4,6)

I believe the answer is:

up ones sleeve

I'm a little stuck... Click here to teach me more about this clue!

'hidden' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'where arm' is the wordplay.
I cannot quite see how this works, but
'arm' could be 'sleeve' (synonyms) and 'sleeve' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

'found' is the link.

Can you help me to learn more?

I've seen this clue in The Telegraph.

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