Sender of message dispatching a runner (5)
I believe the answer is:
miler
'runner' is the definition.
(miler is a kind of runner)
'sender of message dispatching a' is the wordplay.
'sender of message' becomes 'mailer'.
'dispatching' indicates named letters should be taken away.
'mailer' with 'a' taken out is 'MILER'.
(Other definitions for miler that I've seen before include "horse" , "Athlete running 1,760 yards" , "one who competes in the long run" , "Runner over a specific distance" , "Pre-metric middle-distance runner" .)