One dispatching middle-distance runner outside front of arena (6)
I believe the answer is:
mailer
'one dispatching' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'middle-distance runner outside front of arena' is the wordplay.
'middle-distance runner' becomes 'miler' (I've seen this in another clue).
'outside' indicates putting letters inside.
'front of' says to take the initial letters (the letter at the front of the word).
The initial letter of 'arena' is 'a'.
'miler' enclosing 'a' is 'MAILER'.
Can you help me to learn more?
(Other definitions for mailer that I've seen before include "One putting armour on" , "person at the post?" , "Poster" , "American author of The Naked and the Dead, d. 2007" , "writer from America" .)