Quiet runner's shoulder motion (5)
I believe the answer is:
shrug
'shoulder motion' is the definition.
(I've seen this in another clue)
'quiet runner's' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'runner' becomes 'rug' (I've seen this in another clue).
'sh'+'rug'='SHRUG'
(Other definitions for shrug that I've seen before include "Gesture expressing unconcern" , "Raise one's shoulders" , "Gesture of indifference or ignorance" , "Gesture of resignation" , "Momentarily raise the shoulders" .)