Race that can go to an American's head (5)

I believe the answer is:

derby

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'race' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for derby that I've seen before include "Epsom horse-race" , "Canadian bowler" , "... and after 8 [STATE = KENTUCKY] , meeting" , "Race run at Epsom" , "Match between neighbouring teams" .)

I've seen this clue in the Evening Standard.

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