US boxer, undisputed world heavyweight champion 1987-90 (5)

I believe the answer is:

tyson

(Other definitions for tyson that I've seen before include "Undisputed world heavyweight champion, 1987" , "Mike --, US boxer" , "Youngest ever undisputed world heavyweight boxing champion, b, 1966" , "26 4 [RING MASTER]" , "Mike ..., former US prizefighter" .)

I've seen this clue in The Guardian.

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