Fence the track (4)
I believe the answer is:
rail
I believe this is a double definition.
'fence' is the first definition.
(rail fence is a kind of fence)
'the track' is the second definition.
('rail' can be a synonym of 'track')
(Other definitions for rail that I've seen before include "Perch" , "Fulminate - wading bird" , "Form of transport" , "Waterside bird; part of track" , "20 backwards, that 7 runs on" .)