Wear casual clothes, getting reprimand (9)

I believe the answer is:

dressdown

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'reprimand' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'wear casual clothes' is the wordplay.
'wear' becomes 'dress' (both can mean a person's clothing).
'casual clothes' becomes 'down' (I can't explain this - if you can you should give a lot more credence to this answer).
'dress'+'down'='DRESSDOWN'

'getting' acts as a link.

Can you help me to learn more?

I've seen this clue in The Daily Express.

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