Initially high flyers sort of crack (8)

I believe the answer is:

hairline

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'crack' is the definition.
(I have seen 'Thin crack ' mean 'hairline' so perhaps 'crack' could also mean 'hairline')

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for hairline that I've seen before include "A very narrow crack, a little hirsute?" , "Of a crack, very thin" , "(Very fine) crack or fracture" , "Barnet's border" , "Type of crack" .)

I've seen this clue in The Guardian.

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