Short post barring traffic (7)
I believe the answer is:
bollard
'short post barring traffic' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bollard that I've seen before include "Short post barring traffic" , "Strong post" , "Mooring post - traffic restriction device" , "Post on road or quay" , "Thick post for mooring" .)