Crossword Genius

This one was tricky to navigate and did, finally, leave them dumbfounded (6)

Ross

I believe the answer is:

amazed

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'dumbfounded' is the definition.
('amaze' can be a synonym of 'dumbfound')

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for amazed that I've seen before include "Flabbergasted" , "Filled with astonishment" , "Dumbfounded" , "knocked for six" , "Completely surprised" .)

I've seen this clue in The Irish Times.
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