Knock off the track (6)
I believe the answer is:
derail
'knock off the track' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for derail that I've seen before include "put off track" , "Cause to leave the tracks" , "Cause to come off the tracks" , "get off the line" , "go off track" .)