A lost dog is off course (6)
I believe the answer is:
astray
'off course' is the definition.
(to be led astray is to be led off course)
'a lost dog' is the wordplay.
'lost dog' becomes 'stray' (I've seen this in another clue).
'a'+'stray'='ASTRAY'
'is' is the link.
(Other definitions for astray that I've seen before include "Missing, lost" , "Off the correct path" , "Well off course" , "Off target" , "Wandering, off course" .)