Talk about field event, first in series (7)
I believe the answer is:
discuss
'talk about' is the definition.
(to discuss something is to talk about it)
'field event first in series' is the wordplay.
'field event' becomes 'discus' (discus is a kind of field event).
'first in series' becomes 's' (1st letter of 'series').
'discus'+'s'='DISCUSS'
(Other definitions for discuss that I've seen before include "Take up" , "Settle" , "Verbally T [TREAT]" , "Go into" , "Argue" .)