Look happy, having sprinted initial distance (5)
I believe the answer is:
smile
'look happy' is the definition.
(I've seen this in another clue)
'sprinted initial distance' is the wordplay.
'initial' indicates taking the first letters.
'distance' becomes 'mile' (mile is a unit of distance).
The initial letter of 'sprinted' is 's'.
's'+'mile'='SMILE'
'having' is the link.
(Other definitions for smile that I've seen before include "express pleasure" , "Visually express amusement" , "Friendly facial gesture" , "Facial expression showing pleasure" , "Look happy" .)