Racetrack fence (4)
I believe the answer is:
rail
'racetrack fence' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rail that I've seen before include "20 backwards, that 7 runs on" , "Supported bar - inveigh" , "Use abusive language" , "Waterside bird; part of track" , "British --; picture --" .)