Crossword Genius

... idle warning causing depression to Mad Hatter (6,6)

Ross

I believe the answer is:

hollow threat

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

'idle warning' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'depression to mad hatter' is the wordplay.
I cannot quite see how this works, but
'depression' could be 'hollow' (hollow is a kind of depression) and 'hollow' is found in the answer.
an anagram of 'hatter' is 'threat' which is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...

'causing' is the link.

Can you help me to learn more?

I've seen this clue in The Irish Independent.
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