Record set by American in field event (6)
'field event' is the definition.
(discus is a kind of field event)
'record set by american' is the wordplay.
'record set' becomes 'disc' (I've seen this in other clues).
'by' means one lot of letters go next to another.
'american' becomes 'us'.
'in' is the link.
(Other definitions for discus that I've seen before include "Field event projectile", "Circular object thrown in sports", "flat missile", "Athlete's projectile", "Sporting missile".)