Finally, did a quiet run (4)
I believe the answer is:
dash
'run' is the definition.
(I know that dash is a type of track event)
'finally did a quiet' is the wordplay.
'finally' suggests the final letters.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
The last letter of 'did' is 'd'.
'd'+'a'+'sh'='DASH'
(Other definitions for dash that I've seen before include "Splash of colour" , "Stylish vigour; frustrate (hopes)" , "quite mild oath" , "Hurry ("must ____!")" , "Drop" .)