Briefly talk about field event (6)
I believe the answer is:
discus
'field event' is the definition.
(discus is a kind of field event)
'briefly talk about' is the wordplay.
'briefly' means to remove the last letter.
'talk about' becomes 'discuss' (to discuss something is to talk about it).
'discuss' with its last letter taken away is 'DISCUS'.
(Other definitions for discus that I've seen before include "Sports projectile" , "Sporting projectile" , "Circular object thrown in sports" , "Athletic field event" , "Something thrown by athletes" .)