Smoker's receptacle is one-third off course (6)
I believe the answer is:
astray
'off course' is the definition.
(to be led astray is to be led off course)
'smoker's receptacle is one-third' is the wordplay.
I cannot quite understand how this works, but
'receptacle' could be 'tray' (tray is a kind of receptacle) and 'tray' is located in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
A single letter 's' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for astray that I've seen before include "Off-course" , "Off the correct path" , "Well off course" , "Wandering off the path" , "Adrift - misguided - lost" .)