Half a train track (4)
I believe the answer is:
rail
'half a train track' 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 "Supported bar - inveigh" , "Use abusive language" , "Means of transport" , "method of transport" , "Horizontal bar - bird - complain" .)