Track missing its opening bar (4)
I believe the answer is:
rail
This might be a double definition.
'track missing its opening' is the first definition.
I know that 'track' relates to this answer.
'bar' is the second definition.
(I know that rail is a type of bar)
Can you help me to learn more?
(Other definitions for rail that I've seen before include "Bar or banister" , "Track - complain - bird" , "Part of train track" , "Kind of transport" , "Water bird; barrier" .)