Returning home mad and astonished by (7)

I believe the answer is:

stunned

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'astonished by' is the definition.
'stunned' can be an answer for 'astonished' (I've seen this in another clue). I'm not sure about the 'by' bit.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for stunned that I've seen before include "Taken aback" , "Stupefied" , "Amazed" , "Dazed; astonished" , "shock" .)

I've seen this clue in the Evening Standard.

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