Track has one slipping, left abandoned (6)
I believe the answer is:
siding
'track has one' is the definition.
(a railway track has a siding)
'slipping left abandoned' is the wordplay.
'slipping' becomes 'sliding' ('slide' can be a synonym of 'slip').
'left' becomes 'L' (common abbreviation).
'abandoned' suggests deleting specific letters.
'sliding' with 'l' taken away is 'SIDING'.
(Other definitions for siding that I've seen before include "Short stretch of railroad track used for shunting" , "Section of track" , "Taking one part in an argument on the spare railway track" , "Short piece of railway track used for shunting" , "Short stretch of railway track off the main line" .)