Brief note following list of runners (8)
I believe the answer is:
postcard
'brief note' is the definition.
'following list of runners' is the wordplay.
'following' becomes 'post' (eg post-war, post-industrial etc).
'list of runners' becomes 'card' (list of runners at a sporting event).
'post'+'card'='POSTCARD'
(Other definitions for postcard that I've seen before include "Limited communication" , "Letter without an envelope?" , "Brief message" , "Holiday communication" , "Holiday missive" .)