Crossword Genius

Opens act with flourish: a crowning achievement? (8)

Ross

I believe the answer is:

capstone

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

'flourish a crowning achievement?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'opens act' is the wordplay.
'opens act' can be anagrammed to 'CAPSTONE'.
But, I am not sure how this is indicated.

'with' acts as a link.

Can you help me to learn more?

(Other definitions for capstone that I've seen before include "What's put on wall" , "Part of coping" , "Piece on top of wall, etc" .)

I've seen this clue in The Times.
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