Crossword Genius

My tip to thrill in headwear for Ascot? (5,4)

Ross

I believe the answer is:

fancy that

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

'my' is the definition.
(both are expressions of surprise)

'tip to thrill in headwear for ascot?' is the wordplay.
I cannot really understand how this works, but
'headwear' could be 'hat' (hat is a type of headwear) and 'hat' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for fancy that that I've seen before include "Gosh" , "goodness" , "Imagine!" , "exclamation of surprise" , "Well I'm blowed!" .)

I've seen this clue in The Times and The Sunday Times.
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