Those who cannot keep straight go off them (5)
I believe the answer is:
rails
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for rails that I've seen before include "Makes a protest" , "Tracks or barriers" , "Wading birds - dysfunctional person has come off them" , "Barriers of horizontal bars and supports" , "secretive birds" .)